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.
sys.argv[0]
is the script name (it is operating system
dependent whether this is a full pathname or not).
If the command was executed using the `-c' command line option
to the interpreter, sys.argv[0]
is set to the string
"-c"
.
If no script name was passed to the Python interpreter,
sys.argv
has zero length.
sys.modules.keys()
only lists the imported
modules.)
except
clause.'' For any stack frame, only
information about the most recently handled exception is accessible.
If no exception is being handled anywhere on the stack, a tuple
containing three None
values is returned. Otherwise, the
values returned are
(type, value, traceback)
.
Their meaning is: type gets the exception type of the exception
being handled (a string or class object); value gets the
exception parameter (its associated value or the second argument
to raise
, which is always a class instance if the exception
type is a class object); traceback gets a traceback object (see
the Reference Manual) which encapsulates the call stack at the point
where the exception originally occurred.
Warning: assigning the traceback return value to a
local variable in a function that is handling an exception will cause
a circular reference. This will prevent anything referenced by a local
variable in the same function or by the traceback from being garbage
collected. Since most functions don't need access to the traceback,
the best solution is to use something like
type, value = sys.exc_info()[:2]
to extract only the exception type and value. If you do need the
traceback, make sure to delete it after use (best done with a
try-finally
statement) or to call sys.exc_info()
in a
function that does not itself handle an exception.
sys.exc_info()
above. However, since
they are global variables, they are not specific to the current
thread, so their use is not safe in a multi-threaded program. When no
exception is being handled, sys.exc_type
is set to None
and the other two are undefined.
"/usr/local"
. This can be
set at build time with the -exec-prefix
argument to the
configure
script. Specifically, all configuration files
(e.g. the config.h
header file) are installed in the directory
sys.exec_prefix+"/lib/pythonVER/config"
, and shared library
modules are installed in
sys.exec_prefix+"/lib/pythonVER/lib-dynload"
,
where VER is equal to sys.version[:3]
.
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.
import pdb; pdb.pm()
to enter the post-mortem
debugger; see the chapter ``The Python Debugger'' for more
information.)
The meaning of the variables is the same
as that of the return values from sys.exc_info()
above.
(Since there is only one interactive thread, thread-safety is not a
concern for these variables, unlike for sys.exc_type
etc.)
PYTHONPATH
, or an
installation-dependent default.
The first item of this list, sys.path[0]
, is the
directory containing the script that was used to invoke the Python
interpreter. If the script directory is not available (e.g. if the
interpreter is invoked interactively or if the script is read from
standard input), sys.path[0]
is the empty string, which directs
Python to search modules in the current directory first. Notice that
the script directory is inserted before the entries inserted as
a result of $PYTHONPATH
.
sunos5
or
linux1
. This can be used to append platform-specific
components to sys.path
, for instance.
"/usr/local"
. This can be set at build time with the
-prefix
argument to the configure
script. The main
collection of Python library modules is installed in the directory
sys.prefix+"/lib/pythonVER"
while the platform
independent header files (all except config.h
) are stored in
sys.prefix+"/include/pythonVER"
,
where VER is equal to sys.version[:3]
.
'>>> '
and '... '
. If a non-string object is assigned
to either variable, its str()
is re-evaluated each time the
interpreter prepares to read a new interactive command; this can be
used to implement a dynamic prompt.
<=
0 checks every virtual instruction,
maximizing responsiveness as well as overhead.
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
.
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. (Changing these
objects doesn't affect the standard I/O streams of processes
executed by popen()
, system()
or the exec*()
family of functions in the os
module.)