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This is Info file python-lib.info, produced by Makeinfo-1.55 from the input file lib.texi. This file describes the built-in types, exceptions and functions and the standard modules that come with the Python system. It assumes basic knowledge about the Python language. For an informal introduction to the language, see the Python Tutorial. The Python Reference Manual gives a more formal definition of the language. (These manuals are not yet available in INFO or Texinfo format.) Copyright (C) 1991, 1992, 1993, 1994 by Stichting Mathematisch Centrum, Amsterdam, The Netherlands. All Rights Reserved Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the names of Stichting Mathematisch Centrum or CWI not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. STICHTING MATHEMATISCH CENTRUM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. File: python-lib.info, Node: Top, Next: Introduction, Prev: (dir), Up: (dir) The Python library ****************** This file describes the built-in types, exceptions and functions and the standard modules that come with the Python system. It assumes basic knowledge about the Python language. For an informal introduction to the language, see the `Python Tutorial'. The `Python Reference Manual' gives a more formal definition of the language. (These manuals are not yet available in INFO or Texinfo format.) This version corresponds to Python version 1.0.0. * Menu: * Introduction:: * Built-in Objects:: * Built-in Modules:: * Standard Modules:: * UNIX ONLY:: * MULTIMEDIA EXTENSIONS:: * CRYPTOGRAPHIC EXTENSIONS:: * STDWIN ONLY:: * SGI IRIX ONLY:: * SUNOS ONLY:: * Function Index:: * Variable Index:: * Module Index:: * Concept Index:: File: python-lib.info, Node: Introduction, Next: Built-in Objects, Prev: Top, Up: Top Introduction ************ The Python library consists of three parts, with different levels of integration with the interpreter. Closest to the interpreter are built-in types, exceptions and functions. Next are built-in modules, which are written in C and linked statically with the interpreter. Finally there are standard modules that are implemented entirely in Python, but are always available. For efficiency, some standard modules may become built-in modules in future versions of the interpreter. File: python-lib.info, Node: Built-in Objects, Next: Built-in Modules, Prev: Introduction, Up: Top Built-in Types, Exceptions and Functions **************************************** Names for built-in exceptions and functions are found in a separate symbol table. This table is searched last, so local and global user-defined names can override built-in names. Built-in types have no names but are created easily by constructing an object of the desired type (e.g., using a literal) and applying the built-in function `type()' to it. They are described together here for easy reference.(1) * Menu: * Types:: * Exceptions:: * Built-in Functions:: ---------- Footnotes ---------- (1) Some descriptions sorely lack explanations of the exceptions that may be raised -- this will be fixed in a future version of this document. File: python-lib.info, Node: Types, Next: Exceptions, Prev: Built-in Objects, Up: Built-in Objects Built-in Types ============== The following sections describe the standard types that are built into the interpreter. These are the numeric types, sequence types, and several others, including types themselves. There is no explicit Boolean type; use integers instead. Some operations are supported by several object types; in particular, all objects can be compared, tested for truth value, and converted to a string (with the ``...`' notation). The latter conversion is implicitly used when an object is written by the `print' statement. * Menu: * Truth Value Testing:: * Boolean Operations:: * Comparisons:: * Numeric Types:: * Sequence Types:: * Mapping Types:: * Other Built-in Types:: * Special Attributes:: File: python-lib.info, Node: Truth Value Testing, Next: Boolean Operations, Prev: Types, Up: Types Truth Value Testing ------------------- Any object can be tested for truth value, for use in an `if' or `while' condition or as operand of the Boolean operations below. The following values are false: * `None' * zero of any numeric type, e.g., `0', `0L', `0.0'. * any empty sequence, e.g., `''', `()', `[]'. * any empty mapping, e.g., `{}'. *All* other values are true -- so objects of many types are always true. File: python-lib.info, Node: Boolean Operations, Next: Comparisons, Prev: Truth Value Testing, Up: Types Boolean Operations ------------------ These are the Boolean operations: *Operation* *Result* -- *Notes* `X or Y' if X is false, then Y, else X -- (1) `X and Y' if X is false, then X, else Y -- (1) `not X' if X is false, then `1', else `0' Notes: (1) These only evaluate their second argument if needed for their outcome. File: python-lib.info, Node: Comparisons, Next: Numeric Types, Prev: Boolean Operations, Up: Types Comparisons ----------- Comparison operations are supported by all objects: *Operation* *Meaning* -- *Notes* `<' strictly less than `<=' less than or equal `>' strictly greater than `>=' greater than or equal `==' equal `<>' not equal -- (1) `!=' not equal -- (1) `is' object identity `is not' negated object identity Notes: (1) `<>' and `!=' are alternate spellings for the same operator. (I couldn't choose between ABC and C! :-) Objects of different types, except different numeric types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (e.g., windows) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. (Implementation note: objects of different types except numbers are ordered by their type names; objects of the same types that don't support proper comparison are ordered by their address.) Two more operations with the same syntactic priority, `in' and `not in', are supported only by sequence types (below). File: python-lib.info, Node: Numeric Types, Next: Sequence Types, Prev: Comparisons, Up: Types Numeric Types ------------- There are three numeric types: "plain integers", "long integers", and "floating point numbers". Plain integers (also just called "integers") are implemented using `long' in C, which gives them at least 32 bits of precision. Long integers have unlimited precision. Floating point numbers are implemented using `double' in C. All bets on their precision are off unless you happen to know the machine you are working with. Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including hex and octal numbers) yield plain integers. Integer literals with an `L' or `l' suffix yield long integers (`L' is preferred because `1l' looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the "smaller" type is converted to that of the other, where plain integer is smaller than long integer is smaller than floating point. Comparisons between numbers of mixed type use the same rule.(1) The functions `int()', `long()' and `float()' can be used to coerce numbers to a specific type. All numeric types support the following operations: *Operation* *Result* -- *Notes* `abs(X)' absolute value of X `int(X)' X converted to integer -- (1) `long(X)' X converted to long integer -- (1) `float(X)' X converted to floating point `-X' X negated `+X' X unchanged `X + Y' sum of X and Y `X - Y' difference of X and Y `X * Y' product of X and Y `X / Y' quotient of X and Y -- (2) `X % Y' remainder of `X / Y' `divmod(X, Y)' the pair `(X / Y, X % Y)' -- (3) `pow(X, Y)' X to the power Y Notes: (1) Conversion from floating point to (long or plain) integer may round or truncate as in C; see functions `floor' and `ceil' in module `math' for well-defined conversions. (2) For (plain or long) integer division, the result is an integer; it always truncates towards zero. (3) See the section on built-in functions for an exact definition. * Menu: * Bit-string Operations on Integer Types:: ---------- Footnotes ---------- (1) As a consequence, the list `[1, 2]' is considered equal to `[1.0, 2.0]', and similar for tuples. File: python-lib.info, Node: Bit-string Operations on Integer Types, Prev: Numeric Types, Up: Numeric Types Bit-string Operations on Integer Types. ....................................... Plain and long integer types support additional operations that make sense only for bit-strings. Negative numbers are treated as their 2's complement value: *Operation* *Result* -- *Notes* `~X' the bits of X inverted `X ^ Y' bitwise "exclusive or" of X and Y `X & Y' bitwise "and" of X and Y `X | Y' bitwise "or" of X and Y `X << N' X shifted left by N bits `X >> N' X shifted right by N bits File: python-lib.info, Node: Sequence Types, Next: Mapping Types, Prev: Numeric Types, Up: Types Sequence Types -------------- There are three sequence types: strings, lists and tuples. Strings literals are written in single quotes: `'xyzzy''. Lists are constructed with square brackets, separating items with commas: `[a, b, c]'. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, e.g., `a, b, c' or `()'. A single item tuple must have a trailing comma, e.g., `(d,)'. Sequence types support the following operations (S and T are sequences of the same type; N, I and J are integers): *Operation* *Result* -- *Notes* `len(S)' length of S `min(S)' smallest item of S `max(S)' largest item of S `X in S' `1' if an item of S is equal to X, else `0' `X not in S' `0' if an item of S is equal to X, else `1' `S + T' the concatenation of S and T `S * N, N * S' N copies of S concatenated `S[I]' I'th item of S, origin 0 -- (1) `S[I:J]' slice of S from I to J -- (1), (2) Notes: (1) If I or J is negative, the index is relative to the end of the string, i.e., `len(S) + I' or `len(S) + J' is substituted. But note that `-0' is still `0'. (2) The slice of S from I to J is defined as the sequence of items with index K such that `I <= K < J'. If I or J is greater than `len(S)', use `len(S)'. If I is omitted, use `0'. If J is omitted, use `len(S)'. If I is greater than or equal to J, the slice is empty. * Menu: * More String Operations:: * Mutable Sequence Types:: File: python-lib.info, Node: More String Operations, Next: Mutable Sequence Types, Prev: Sequence Types, Up: Sequence Types More String Operations. ....................... String objects have one unique built-in operation: the `%' operator (modulo) with a string left argument interprets this string as a C sprintf format string to be applied to the right argument, and returns the string resulting from this formatting operation. Unless the format string requires exactly one argument, the right argument should be a tuple of the correct size. The following format characters are understood: %, c, s, i, d, u, o, x, X, e, E, f, g, G. Width and precision may be a * to specify that an integer argument specifies the actual width or precision. The flag characters -, +, blank, # and 0 are understood. The size specifiers h, l or L may be present but are ignored. The ANSI features `%p' and `%n' are not supported. Since Python strings have an explicit length, `%s' conversions don't assume that `' 0'' is the end of the string. For safety reasons, huge floating point precisions are truncated; `%f' conversions for huge numbers are replaced by `%g' conversions. All other errors raise exceptions. Additional string operations are defined in standard module `string' and in built-in module `regex'. File: python-lib.info, Node: Mutable Sequence Types, Prev: More String Operations, Up: Sequence Types Mutable Sequence Types. ....................... List objects support additional operations that allow in-place modification of the object. These operations would be supported by other mutable sequence types (when added to the language) as well. Strings and tuples are immutable sequence types and such objects cannot be modified once created. The following operations are defined on mutable sequence types (where X is an arbitrary object): *Operation* *Result* -- *Notes* `S[I] = X' item I of S is replaced by X `S[I:J] = T' slice of S from I to J is replaced by T `del S[I:J]' same as `S[I:J] = []' `S.append(X)' same as `S[len(X):len(X)] = [X]' `S.count(X)' return number of I's for which `S[I] == X' `S.index(X)' return smallest I such that `S[I] == X' -- (1) `S.insert(I, X)' same as `S[I:I] = [X]' `S.remove(X)' same as `del S[S.index(X)]' -- (1) `S.reverse()' reverses the items of S in place `S.sort()' permutes the items of S to satisfy `S[I] <= S[J]', for `I < J' -- (2) Notes: (1) Raises an exception when X is not found in S. (2) The `sort()' method takes an optional argument specifying a comparison function of two arguments (list items) which should return `-1', `0' or `1' depending on whether the first argument is considered smaller than, equal to, or larger than the second argument. Note that this slows the sorting process down considerably; e.g. to sort an array in reverse order it is much faster to use calls to `sort()' and `reverse()' than to use `sort()' with a comparison function that reverses the ordering of the elements. File: python-lib.info, Node: Mapping Types, Next: Other Built-in Types, Prev: Sequence Types, Up: Types Mapping Types ------------- A "mapping" object maps values of one type (the key type) to arbitrary objects. Mappings are mutable objects. There is currently only one mapping type, the "dictionary". A dictionary's keys are almost arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g. 1 and 1.0) then they can be used interchangeably to index the same dictionary entry. Dictionaries are created by placing a comma-separated list of `KEY: VALUE' pairs within braces, for example: `{'jack': 4098, 'sjoerd: 4127}' or `{4098: 'jack', 4127: 'sjoerd}'. The following operations are defined on mappings (where A is a mapping, K is a key and X is an arbitrary object): *Operation* *Result* -- *Notes* `len(A)' the number of items in A `A[K]' the item of A with key K -- (1) `A[K] = X' set `A[K]' to X `del A[K]' remove `A[K]' from A -- (1) `A.items()' a copy of A's list of (key, item) pairs -- (2) `A.keys()' a copy of A's list of keys -- (2) `A.values()' a copy of A's list of values -- (2) `A.has_key(K)' `1' if A has a key K, else `0' Notes: (1) Raises an exception if K is not in the map. (2) Keys and values are listed in random order, but at any moment the ordering of the `keys()', `values()' and `items()' lists is the consistent with each other. File: python-lib.info, Node: Other Built-in Types, Next: Special Attributes, Prev: Mapping Types, Up: Types Other Built-in Types -------------------- The interpreter supports several other kinds of objects. Most of these support only one or two operations. * Menu: * Modules:: * Classes and Class Instances:: * Functions:: * Methods:: * Type Objects:: * The Null Object:: * File Objects:: * Internal Objects:: File: python-lib.info, Node: Modules, Next: Classes and Class Instances, Prev: Other Built-in Types, Up: Other Built-in Types Modules. ........ The only special operation on a module is attribute access: `M.NAME', where M is a module and NAME accesses a name defined in M's symbol table. Module attributes can be assigned to. (Note that the `import' statement is not, strictly spoken, an operation on a module object; `import FOO' does not require a module object named FOO to exist, rather it requires an (external) *definition* for a module named FOO somewhere.) A special member of every module is `__dict__'. This is the dictionary containing the module's symbol table. Modifying this dictionary will actually change the module's symbol table, but direct assignment to the `__dict__' attribute is not possible (i.e., you can write `M.__dict__['a'] = 1', which defines `M.a' to be `1', but you can't write `M.__dict__ = {}'. Modules are written like this: `<module 'sys'>'. File: python-lib.info, Node: Classes and Class Instances, Next: Functions, Prev: Modules, Up: Other Built-in Types Classes and Class Instances. ............................ (See the Python Reference Manual for these.) File: python-lib.info, Node: Functions, Next: Methods, Prev: Classes and Class Instances, Up: Other Built-in Types Functions. .......... Function objects are created by function definitions. The only operation on a function object is to call it: `FUNC(ARGUMENT-LIST)'. There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types. The implementation adds two special read-only attributes: `F.func_code' is a function's "code object" (see below) and `F.func_globals' is the dictionary used as the function's global name space (this is the same as `M.__dict__' where M is the module in which the function F was defined). File: python-lib.info, Node: Methods, Next: Type Objects, Prev: Functions, Up: Other Built-in Types Methods. ........ Methods are functions that are called using the attribute notation. There are two flavors: built-in methods (such as `append()' on lists) and class instance methods. Built-in methods are described with the types that support them. The implementation adds two special read-only attributes to class instance methods: `M.im_self' is the object whose method this is, and `M.im_func' is the function implementing the method. Calling `M(ARG-1, ARG-2, ..., ARG-N)' is completely equivalent to calling `M.im_func(M.im_self, ARG-1, ARG-2, ..., ARG-N)'. (See the Python Reference Manual for more info.) File: python-lib.info, Node: Type Objects, Next: The Null Object, Prev: Methods, Up: Other Built-in Types Type Objects. ............. Type objects represent the various object types. An object's type is accessed by the built-in function `type()'. There are no special operations on types. Types are written like this: `<type 'int'>'. File: python-lib.info, Node: The Null Object, Next: File Objects, Prev: Type Objects, Up: Other Built-in Types The Null Object. ................ This object is returned by functions that don't explicitly return a value. It supports no special operations. There is exactly one null object, named `None' (a built-in name). It is written as `None'. File: python-lib.info, Node: File Objects, Next: Internal Objects, Prev: The Null Object, Up: Other Built-in Types File Objects. ............. File objects are implemented using C's `stdio' package and can be created with the built-in function `open()' described under Built-in Functions below. When a file operation fails for an I/O-related reason, the exception `IOError' is raised. This includes situations where the operation is not defined for some reason, like `seek()' on a tty device or writing a file opened for reading. Files have the following methods: - Method on file: close () Close the file. A closed file cannot be read or written anymore. - Method on file: flush () Flush the internal buffer, like `stdio''s `fflush()'. - Method on file: isatty () Return `1' if the file is connected to a tty(-like) device, else `0'. - Method on file: read (SIZE) Read at most SIZE bytes from the file (less if the read hits EOF or no more data is immediately available on a pipe, tty or similar device). If the SIZE argument is omitted, read all data until EOF is reached. The bytes are returned as a string object. An empty string is returned when EOF is encountered immediately. (For certain files, like ttys, it makes sense to continue reading after an EOF is hit.) - Method on file: readline () Read one entire line from the file. A trailing newline character is kept in the string (but may be absent when a file ends with an incomplete line). An empty string is returned when EOF is hit immediately. Note: unlike `stdio''s `fgets()', the returned string contains null characters (`'\0'') if they occurred in the input. - Method on file: readlines () Read until EOF using `readline()' and return a list containing the lines thus read. - Method on file: seek (OFFSET, WHENCE) Set the file's current position, like `stdio''s `fseek()'. The WHENCE argument is optional and defaults to `0' (absolute file positioning); other values are `1' (seek relative to the current position) and `2' (seek relative to the file's end). There is no return value. - Method on file: tell () Return the file's current position, like `stdio''s `ftell()'. - Method on file: write (STR) Write a string to the file. There is no return value. File: python-lib.info, Node: Internal Objects, Prev: File Objects, Up: Other Built-in Types Internal Objects. ................. (See the Python Reference Manual for these.) File: python-lib.info, Node: Special Attributes, Prev: Other Built-in Types, Up: Types Special Attributes ------------------ The implementation adds a few special read-only attributes to several object types, where they are relevant: * `X.__dict__' is a dictionary of some sort used to store an object's (writable) attributes; * `X.__methods__' lists the methods of many built-in object types, e.g., `[].__methods__' is `['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']'; * `X.__members__' lists data attributes; * `X.__class__' is the class to which a class instance belongs; * `X.__bases__' is the tuple of base classes of a class object. File: python-lib.info, Node: Exceptions, Next: Built-in Functions, Prev: Types, Up: Built-in Objects Built-in Exceptions =================== Exceptions are string objects. Two distinct string objects with the same value are different exceptions. This is done to force programmers to use exception names rather than their string value when specifying exception handlers. The string value of all built-in exceptions is their name, but this is not a requirement for user-defined exceptions or exceptions defined by library modules. The following exceptions can be generated by the interpreter or built-in functions. Except where mentioned, they have an `associated value' indicating the detailed cause of the error. This may be a string or a tuple containing several items of information (e.g., an error code and a string explaining the code). User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition `just like' the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error. - built-in exception: AttributeError Raised when an attribute reference or assignment fails. (When an object does not support attributes references or attribute assignments at all, `TypeError' is raised.) - built-in exception: EOFError Raised when one of the built-in functions (`input()' or `raw_input()') hits an end-of-file condition (EOF) without reading any data. (N.B.: the `read()' and `readline()' methods of file objects return an empty string when they hit EOF.) No associated value. - built-in exception: IOError Raised when an I/O operation (such as a `print' statement, the built-in `open()' function or a method of a file object) fails for an I/O-related reason, e.g., `file not found', `disk full'. - built-in exception: ImportError Raised when an `import' statement fails to find the module definition or when a `from ... import' fails to find a name that is to be imported. - built-in exception: IndexError Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is not a plain integer, `TypeError' is raised.) - built-in exception: KeyError Raised when a mapping (dictionary) key is not found in the set of existing keys. - built-in exception: KeyboardInterrupt Raised when the user hits the interrupt key (normally `Control-C' or DEL). During execution, a check for interrupts is made regularly. Interrupts typed when a built-in function `input()' or `raw_input()') is waiting for input also raise this exception. No associated value. - built-in exception: MemoryError Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects). The associated value is a string indicating what kind of (internal) operation ran out of memory. Note that because of the underlying memory management architecture (C's `malloc()' function), the interpreter may not always be able to completely recover from this situation; it nevertheless raises an exception so that a stack traceback can be printed, in case a run-away program was the cause. - built-in exception: NameError Raised when a local or global name is not found. This applies only to unqualified names. The associated value is the name that could not be found. - built-in exception: OverflowError Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for long integers (which would rather raise `MemoryError' than give up). Because of the lack of standardization of floating point exception handling in C, most floating point operations also aren't checked. For plain integers, all operations that can overflow are checked except left shift, where typical applications prefer to drop bits than raise an exception. - built-in exception: RuntimeError Raised when an error is detected that doesn't fall in any of the other categories. The associated value is a string indicating what precisely went wrong. (This exception is a relic from a previous version of the interpreter; it is not used any more except by some extension modules that haven't been converted to define their own exceptions yet.) - built-in exception: SyntaxError Raised when the parser encounters a syntax error. This may occur in an `import' statement, in an `exec' statement, in a call to the built-in function `eval()' or `input()', or when reading the initial script or standard input (also interactively). - built-in exception: SystemError Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms). You should report this to the author or maintainer of your Python interpreter. Be sure to report the version string of the Python interpreter (`sys.version'; it is also printed at the start of an interactive Python session), the exact error message (the exception's associated value) and if possible the source of the program that triggered the error. - built-in exception: SystemExit This exception is raised by the `sys.exit()' function. When it is not handled, the Python interpreter exits; no stack traceback is printed. If the associated value is a plain integer, it specifies the system exit status (passed to C's `exit()' function); if it is `None', the exit status is zero; if it has another type (such as a string), the object's value is printed and the exit status is one. A call to `sys.exit' is translated into an exception so that clean-up handlers (`finally' clauses of `try' statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The `posix._exit()' function can be used if it is absolutely positively necessary to exit immediately (e.g., after a `fork()' in the child process). - built-in exception: TypeError Raised when a built-in operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch. - built-in exception: ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as `IndexError'. - built-in exception: ZeroDivisionError Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation. File: python-lib.info, Node: Built-in Functions, Prev: Exceptions, Up: Built-in Objects Built-in Functions ================== The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order. - built-in function: abs (X) Return the absolute value of a number. The argument may be a plain or long integer or a floating point number. - built-in function: apply (FUNCTION, ARGS) The FUNCTION argument must be a callable object (a user-defined or built-in function or method, or a class object) and the ARGS argument must be a tuple. The FUNCTION is called with ARGS as argument list; the number of arguments is the the length of the tuple. (This is different from just calling `FUNC(ARGS)', since in that case there is always exactly one argument.) - built-in function: chr (I) Return a string of one character whose ASCII code is the integer I, e.g., `chr(97)' returns the string `'a''. This is the inverse of `ord()'. The argument must be in the range [0..255], inclusive. - built-in function: cmp (X, Y) Compare the two objects X and Y and return an integer according to the outcome. The return value is negative if `X < Y', zero if `X == Y' and strictly positive if `X > Y'. - built-in function: coerce (X, Y) Return a tuple consisting of the two numeric arguments converted to a common type, using the same rules as used by arithmetic operations. - built-in function: compile (STRING, FILENAME, KIND) Compile the STRING into a code object. Code objects can be executed by a `exec()' statement or evaluated by a call to `eval()'. The FILENAME argument should give the file from which the code was read; pass e.g. `'<string>'' if it wasn't read from a file. The KIND argument specifies what kind of code must be compiled; it can be `'exec'' if STRING consists of a sequence of statements, or `'eval'' if it consists of a single expression. - built-in function: dir () Without arguments, return the list of names in the current local symbol table. With a module, class or class instance object as argument (or anything else that has a `__dict__' attribute), returns the list of names in that object's attribute dictionary. The resulting list is sorted. For example: >>> import sys >>> dir() ['sys'] >>> dir(sys) ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout'] >>> - built-in function: divmod (A, B) Take two numbers as arguments and return a pair of integers consisting of their integer quotient and remainder. With mixed operand types, the rules for binary arithmetic operators apply. For plain and long integers, the result is the same as `(A / B, A % B)'. For floating point numbers the result is the same as `(math.floor(A / B), A % B)'. - built-in function: eval (S, GLOBALS, LOCALS) The arguments are a string and two optional dictionaries. The string argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using the dictionaries as global and local name space. The string must not contain null bytes or newline characters. The return value is the result of the expression. If the third argument is omitted it defaults to the second. If both dictionaries are omitted, the expression is executed in the environment where `eval' is called. Syntax errors are reported as exceptions. Example: >>> x = 1 >>> print eval('x+1') 2 >>> This function can also be used to execute arbitrary code objects (e.g. created by `compile()'). In this case pass a code object instead of a string. The code object must have been compiled passing `'eval'' to the KIND argument. Note: dynamic execution of statements is supported by the `exec' statement. - built-in function: filter (FUNCTION, LIST) Construct a list from those elements of LIST for which FUNCTION returns true. If LIST is a string or a tuple, the result also has that type; otherwise it is always a list. If FUNCTION is `None', the identity function is assumed, i.e. all elements of LIST that are false (zero or empty) are removed. - built-in function: float (X) Convert a number to floating point. The argument may be a plain or long integer or a floating point number. - built-in function: getattr (OBJECT, NAME) The arguments are an object and a string. The string must be the name of one of the object's attributes. The result is the value of that attribute. For example, `getattr(X, 'FOOBAR')' is equivalent to `X.FOOBAR'. - built-in function: hasattr (OBJECT, NAME) The arguments are an object and a string. The result is 1 if the string is the name of one of the object's attributes, 0 if not. (This is implemented by calling `getattr(object, name)' and seeing whether it raises an exception or not.) - built-in function: hash (OBJECT) Return the hash value of the object (if it has one). Hash values are 32-bit integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, e.g. 1 and 1.0). - built-in function: hex (X) Convert a number to a hexadecimal string. The result is a valid Python expression. - built-in function: id (OBJECT) Return the `identity' of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. (Two objects whose lifetimes are disjunct may have the same id() value.) (Implementation note: this is the address of the object.) - built-in function: input (PROMPT) Almost equivalent to `eval(raw_input(PROMPT))'. As for `raw_input()', the prompt argument is optional. The difference is that a long input expression may be broken over multiple lines using the backslash convention. - built-in function: int (X) Convert a number to a plain integer. The argument may be a plain or long integer or a floating point number. - built-in function: len (S) Return the length (the number of items) of an object. The argument may be a sequence (string, tuple or list) or a mapping (dictionary). - built-in function: long (X) Convert a number to a long integer. The argument may be a plain or long integer or a floating point number. - built-in function: map (FUNCTION, LIST, ...) Apply FUNCTION to every item of LIST and return a list of the results. If additional LIST arguments are passed, FUNCTION must take that many arguments and is applied to the items of all lists in parallel; if a list is shorter than another it is assumed to be extended with `None' items. If FUNCTION is `None', the identity function is assumed; if there are multiple list arguments, `map' returns a list consisting of tuples containing the corresponding items from all lists (i.e. a kind of transpose operation). The LIST arguments may be any kind of sequence; the result is always a list. - built-in function: max (S) Return the largest item of a non-empty sequence (string, tuple or list). - built-in function: min (S) Return the smallest item of a non-empty sequence (string, tuple or list). - built-in function: oct (X) Convert a number to an octal string. The result is a valid Python expression. - built-in function: open (FILENAME, MODE) Return a new file object (described earlier under Built-in Types). The string arguments are the same as for `stdio''s `fopen()': FILENAME is the file name to be opened, MODE indicates how the file is to be opened: `'r'' for reading, `'w'' for writing (truncating an existing file), and `'a'' opens it for appending. Modes `'r+'', `'w+'' and `'a+'' open the file for updating, provided the underlying `stdio' library understands this. On systems that differentiate between binary and text files, `'b'' appended to the mode opens the file in binary mode. If the file cannot be opened, `IOError' is raised. - built-in function: ord (C) Return the ASCII value of a string of one character. E.g., `ord('a')' returns the integer `97'. This is the inverse of `chr()'. - built-in function: pow (X, Y) Return X to the power Y. The arguments must have numeric types. With mixed operand types, the rules for binary arithmetic operators apply. The effective operand type is also the type of the result; if the result is not expressible in this type, the function raises an exception; e.g., `pow(2, -1)' is not allowed. - built-in function: range (START, END, STEP) This is a versatile function to create lists containing arithmetic progressions. It is most often used in `for' loops. The arguments must be plain integers. If the STEP argument is omitted, it defaults to `1'. If the START argument is omitted, it defaults to `0'. The full form returns a list of plain integers `[START, START + STEP, START + 2 * STEP, ...]'. If STEP is positive, the last element is the largest `START + I * STEP' less than END; if STEP is negative, the last element is the largest `START + I * STEP' greater than END. STEP must not be zero. Example: >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(1, 11) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> range(0, 30, 5) [0, 5, 10, 15, 20, 25] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(0, -10, -1) [0, -1, -2, -3, -4, -5, -6, -7, -8, -9] >>> range(0) [] >>> range(1, 0) [] >>> - built-in function: raw_input (PROMPT) The string argument is optional; if present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, `EOFError' is raised. Example: >>> s = raw_input('--> ') --> Monty Python's Flying Circus >>> s 'Monty Python\'s Flying Circus' >>> - built-in function: reduce (FUNCTION, LIST, INITIALIZER) Apply the binary FUNCTION to the items of LIST so as to reduce the list to a single value. E.g., `reduce(lambda x, y: x*y, LIST, 1)' returns the product of the elements of LIST. The optional INITIALIZER can be thought of as being prepended to LIST so as to allow reduction of an empty LIST. The LIST arguments may be any kind of sequence. - built-in function: reload (MODULE) Re-parse and re-initialize an already imported MODULE. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. Note that if a module is syntactically correct but its initialization fails, the first `import' statement for it does not import the name, but does create a (partially initialized) module object; to reload the module you must first `import' it again (this will just make the partially initialized module object available) before you can `reload()' it. - built-in function: repr (OBJECT) Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to `eval()'. - built-in function: round (X, N) Return the floating point value X rounded to N digits after the decimal point. If N is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus N; if two multiples are equally close, rounding is done away from 0 (so e.g. `round(0.5)' is `1.0' and `round(-0.5)' is `-1.0'). - built-in function: setattr (OBJECT, NAME, VALUE) This is the counterpart of `getattr'. The arguments are an object, a string and an arbitrary value. The string must be the name of one of the object's attributes. The function assigns the value to the attribute, provided the object allows it. For example, `setattr(X, 'FOOBAR', 123)' is equivalent to `X.FOOBAR = 123'. - built-in function: str (OBJECT) Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with `repr(OBJECT' is that `str(OBJECT' does not always attempt to return a string that is acceptable to `eval()'; its goal is to return a printable string. - built-in function: type (OBJECT) Return the type of an OBJECT. The return value is a type object. There is not much you can do with type objects except compare them to other type objects; e.g., the following checks if a variable is a string: >>> if type(x) == type(''): print 'It is a string' File: python-lib.info, Node: Built-in Modules, Next: Standard Modules, Prev: Built-in Objects, Up: Top Built-in Modules **************** The modules described in this chapter are built into the interpreter and considered part of Python's standard environment: they are always avaialble.(1) * Menu: * sys:: * __builtin__:: * __main__:: * array:: * math:: * time:: * regex:: * marshal:: * struct:: ---------- Footnotes ---------- (1) at least in theory -- it is possible to specify at build time that one or more of these modules should be excluded, but it would be antisocial to do so. File: python-lib.info, Node: sys, Next: __builtin__, Prev: Built-in Modules, Up: Built-in Modules Built-in Module `sys' ===================== This module provides access to some variables used or maintained by the interpreter and to functions that interact strongly with the interpreter. It is always available. - data of module sys: argv The list of command line arguments passed to a Python script. `sys.argv[0]' is the script name. If no script name was passed to the Python interpreter, `sys.argv' is empty. - data of module sys: builtin_module_names A list of strings giving the names of all modules that are compiled into this Python interpreter. (This information is not available in any other way -- `sys.modules.keys()' only lists the imported modules.) - data of module sys: exc_type - data of module sys: exc_value - data of module sys: exc_traceback These three variables are not always defined; they are set when an exception handler (an `except' clause of a `try' statement) is invoked. Their meaning is: `exc_type' gets the exception type of the exception being handled; `exc_value' gets the exception parameter (its "associated value" or the second argument to `raise'); `exc_traceback' gets a traceback object which encapsulates the call stack at the point where the exception originally occurred. - function of module sys: exit (N) Exit from Python with numeric exit status N. This is implemented by raising the `SystemExit' exception, so cleanup actions specified by `finally' clauses of `try' statements are honored, and it is possible to catch the exit attempt at an outer level. - data of module sys: exitfunc This value is not actually defined by the module, but can be set by the user (or by a program) to specify a clean-up action at program exit. When set, it should be a parameterless function. This function will be called when the interpreter exits in any way (but not when a fatal error occurs: in that case the interpreter's internal state cannot be trusted). - data of module sys: last_type - data of module sys: last_value - data of module sys: last_traceback These three variables are not always defined; they are set when an exception is not handled and the interpreter prints an error message and a stack traceback. Their intended use is to allow an interactive user to import a debugger module and engage in post-mortem debugging without having to re-execute the command that cause the error (which may be hard to reproduce). The meaning of the variables is the same as that of `exc_type', `exc_value' and `exc_tracaback', respectively. - data of module sys: modules Gives the list of modules that have already been loaded. This can be manipulated to force reloading of modules and other tricks. - data of module sys: path A list of strings that specifies the search path for modules. Initialized from the environment variable `PYTHONPATH', or an installation-dependent default. - data of module sys: ps1 - data of module sys: ps2 Strings specifying the primary and secondary prompt of the interpreter. These are only defined if the interpreter is in interactive mode. Their initial values in this case are `'>>> '' and `'... ''. - function of module sys: settrace (TRACEFUNC) Set the system's trace function, which allows you to implement a Python source code debugger in Python. The standard modules `pdb' and `wdb' are such debuggers; the difference is that `wdb' uses windows and needs STDWIN, while `pdb' has a line-oriented interface not unlike dbx. See the file `pdb.doc' in the Python library source directory for more documentation (both about `pdb' and `sys.trace'). - function of module sys: setprofile (PROFILEFUNC) Set the system's profile function, which allows you to implement a Python source code profiler in Python. The system's profile function is called similarly to the system's trace function (see `sys.settrace'), but it isn't called for each executed line of code (only on call and return and when an exception occurs). Also, its return value is not used, so it can just return `None'. - data of module sys: stdin - data of module sys: stdout - data of module sys: stderr File objects corresponding to the interpreter's standard input, output and error streams. `sys.stdin' is used for all interpreter input except for scripts but including calls to `input()' and `raw_input()'. `sys.stdout' is used for the output of `print' and expression statements and for the prompts of `input()' and `raw_input()'. The interpreter's own prompts and (almost all of) its error messages go to `sys.stderr'. `sys.stdout' and `sys.stderr' needn't be built-in file objects: any object is acceptable as long as it has a `write' method that takes a string argument. - data of module sys: tracebacklimit When this variable is set to an integer value, it determines the maximum number of levels of traceback information printed when an unhandled exception occurs. The default is 1000. When set to 0 or less, all traceback information is suppressed and only the exception type and value are printed. File: python-lib.info, Node: __builtin__, Next: __main__, Prev: sys, Up: Built-in Modules Built-in Module `__builtin__' ============================= This module provides direct access to all `built-in' identifier of Python; e.g. `__builtin__.open' is the full name for the built-in function `open'. File: python-lib.info, Node: __main__, Next: array, Prev: __builtin__, Up: Built-in Modules Built-in Module `__main__' ========================== This module represents the (otherwise anonymous) scope in which the interpreter's main program executes -- commands read either from standard input or from a script file.