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Python Library Reference |  |
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Python Library Reference | |
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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.
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, argv[0] is set to the string
'-c'.
If no script name was passed to the Python interpreter,
argv has zero length.
'big' on big-endian (most-signigicant byte first) platforms,
and 'little' on little-endian (least-significant byte first)
platforms.
New in version 2.0.
modules.keys() only lists the imported
modules.)
None, this function prints it to
sys.stdout, and saves it in __builtin__._.
sys.displayhook is called on the result of evaluating
an expression entered in an interactive Python session.
The display of these values can be customized by assigning
another one-argument function to sys.displayhook.
sys.stderr.
When an exception is raised and uncaught, the interpreter calls
sys.excepthook with three arguments, the exception class,
exception instance, and a traceback object.
In an interactive session this happens just before
control is returned to the prompt; in a Python program this happens
just before the program exits.
The handling of such top-level exceptions can be customized by
assigning another three-argument function to sys.excepthook.
displayhook
and excepthook at the start of the program. They are saved
so that displayhook and excepthook can be restored
in case they happen to get replaced with broken objects.
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
exctype, 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
exc_info() in a function that does not itself handle an
exception.
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, 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
exec_prefix + '/lib/pythonversion/config', and shared
library modules are installed in exec_prefix +
'/lib/pythonversion/lib-dynload', where version is equal
to version[:3].
None is equivalent to passing zero, and any other
object is printed to sys.stderr and results in an exit code of
1. In particular, sys.exit("some error message") is a quick
way to exit a program when an error occurs.
os._exit() is called.
This function should be used for internal and specialized purposes only.
if sys.hexversion >= 0x010502F0:
# use some advanced feature
...
else:
# use an alternative implementation or warn the user
...
This is called "hexversion" since it only really looks meaningful
when viewed as the result of passing it to the built-in
hex() function. The version_info value may be used
for a more human-friendly encoding of the same information.
New in version 1.5.2.
The meaning of the variables is the same
as that of the return values from exc_info() above.
(Since there is only one interactive thread, thread-safety is not a
concern for these variables, unlike for exc_type etc.)
-maxint-1 - the asymmetry results from the use of 2's
complement binary arithmetic.
The first item of this list, 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), 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 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 prefix +
'/lib/pythonversion' while the platform independent header
files (all except config.h) are stored in prefix +
'/include/pythonversion', where version is equal to
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.
10, meaning
the check is performed every 10 Python virtual instructions. Setting
it to a larger value may increase performance for programs using
threads. Setting it to a value <= 0 checks every virtual instruction,
maximizing responsiveness as well as overhead.
None.
The highest possible limit is platform-dependent. A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.
stdin is used for all
interpreter input except for scripts but including calls to
input() and
raw_input() . 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
stderr. stdout and 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 os.popen(), os.system() or the
exec*() family of functions in the os module.)
stdin,
stderr and stdout at the start of the program. They are
used during finalization, and could be useful to restore the actual
files to known working file objects in case they have been overwritten
with a broken object.
1000. When set to
0 or less, all traceback information is suppressed and only the
exception type and value are printed.
'version (#build_number,
build_date, build_time) [compiler]'. The first
three characters are used to identify the version in the installation
directories (where appropriate on each platform). An example:
>>> import sys >>> sys.version '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
'alpha', 'beta',
'candidate', or 'final'. The version_info value
corresponding to the Python version 2.0 is
(2, 0, 0, 'final', 0).
New in version 2.0.
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Python Library Reference | |
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