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(python2.1-lib.info)sys
System-specific parameters and functions
========================================
Access system-specific parameters and functions.
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'
The list of command line arguments passed to a Python script.
`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.
`byteorder'
An indicator of the native byte order. This will have the value
`'big'' on big-endian (most-signigicant byte first) platforms, and
`'little'' on little-endian (least-significant byte first)
platforms. _Added in Python version 2.0_
`builtin_module_names'
A tuple of strings giving the names of all modules that are
compiled into this Python interpreter. (This information is not
available in any other way -- `modules.keys()' only lists the
imported modules.)
`copyright'
A string containing the copyright pertaining to the Python
interpreter.
`dllhandle'
Integer specifying the handle of the Python DLL. Availability:
Windows.
`displayhook(VALUE)'
If VALUE is not `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'.
`excepthook(TYPE, VALUE, TRACEBACK)'
This function prints out a given traceback and exception to
`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__'
`__excepthook__'
These objects contain the original values of `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.
`exc_info()'
This function returns a tuple of three values that give information
about the exception that is currently being handled. The
information returned is specific both to the current thread and to
the current stack frame. If the current stack frame is not
handling an exception, the information is taken from the calling
stack frame, or its caller, and so on until a stack frame is found
that is handling an exception. Here, "handling an exception" is
defined as "executing or having executed an 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 `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.
`exc_type'
`exc_value'
`exc_traceback'
_This is deprecated in Python 1.5. Use `exc_info()' instead._
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.
`exec_prefix'
A string giving the site-specific directory prefix where the
platform-dependent Python files are installed; by default, this is
also `'/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]'.
`executable'
A string giving the name of the executable binary for the Python
interpreter, on systems where this makes sense.
`exit([arg])'
Exit from Python. 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 intercept the
exit attempt at an outer level. The optional argument ARG can be
an integer giving the exit status (defaulting to zero), or another
type of object. If it is an integer, zero is considered
"successful termination" and any nonzero value is considered
"abnormal termination" by shells and the like. Most systems
require it to be in the range 0-127, and produce undefined results
otherwise. Some systems have a convention for assigning specific
meanings to specific exit codes, but these are generally
underdeveloped; Unix programs generally use 2 for command line
syntax errors and 1 for all other kind of errors. If another type
of object is passed, `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.
`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. Only one
function may be installed in this way; to allow multiple functions
which will be called at termination, use the `atexit' module.
Note: the exit function is not called when the program is killed
by a signal, when a Python fatal internal error is detected, or
when `os._exit()' is called.
`getdefaultencoding()'
Return the name of the current default string encoding used by the
Unicode implementation. _Added in Python version 2.0_
`getrefcount(object)'
Return the reference count of the OBJECT. The count returned is
generally one higher than you might expect, because it includes the
(temporary) reference as an argument to `getrefcount()'.
`getrecursionlimit()'
Return the current value of the recursion limit, the maximum depth
of the Python interpreter stack. This limit prevents infinite
recursion from causing an overflow of the C stack and crashing
Python. It can be set by `setrecursionlimit()'.
`_getframe([depth])'
Return a frame object from the call stack. If optional integer
DEPTH is given, return the frame object that many calls below the
top of the stack. If that is deeper than the call stack,
`ValueError' is raised. The default for DEPTH is zero, returning
the frame at the top of the call stack.
This function should be used for internal and specialized purposes
only.
`hexversion'
The version number encoded as a single integer. This is
guaranteed to increase with each version, including proper support
for non-production releases. For example, to test that the Python
interpreter is at least version 1.5.2, use:
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. _Added in Python
version 1.5.2_
`last_type'
`last_value'
`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 caused the error. (Typical use is `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 `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'
The largest positive integer supported by Python's regular integer
type. This is at least 2**31-1. The largest negative integer is
`-maxint-1' - the asymmetry results from the use of 2's complement
binary arithmetic.
`modules'
This is a dictionary that maps module names to modules which have
already been loaded. This can be manipulated to force reloading of
modules and other tricks. Note that removing a module from this
dictionary is _not_ the same as calling `reload()' on the
corresponding module object.
`path'
A list of strings that specifies the search path for modules.
Initialized from the environment variable `PYTHONPATH', or an
installation-dependent default.
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'.
`platform'
This string contains a platform identifier, e.g. `'sunos5'' or
`'linux1''. This can be used to append platform-specific
components to `path', for instance.
`prefix'
A string giving the site-specific directory prefix where the
platform independent Python files are installed; by default, this
is the string `'/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]'.
`ps1'
`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 `'... ''. 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.
`setcheckinterval(interval)'
Set the interpreter's "check interval". This integer value
determines how often the interpreter checks for periodic things
such as thread switches and signal handlers. The default is `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.
`setdefaultencoding(name)'
Set the current default string encoding used by the Unicode
implementation. If NAME does not match any available encoding,
`LookupError' is raised. This function is only intended to be
used by the `site' module implementation and, where needed, by
`sitecustomize'. Once used by the `site' module, it is removed
from the `sys' module's namespace. _Added in Python version 2.0_
`setprofile(profilefunc)'
Set the system's profile function, which allows you to implement a
Python source code profiler in Python. See the chapter on the
Python Profiler. The system's profile function is called
similarly to the system's trace function (see `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'.
`setrecursionlimit(limit)'
Set the maximum depth of the Python interpreter stack to LIMIT.
This limit prevents infinite recursion from causing an overflow of
the C stack and crashing Python.
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.
`settrace(tracefunc)'
Set the system's trace function, which allows you to implement a
Python source code debugger in Python. See section "How It Works"
in the chapter on the Python Debugger.
`stdin'
`stdout'
`stderr'
File objects corresponding to the interpreter's standard input,
output and error streams. `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__'
`__stdout__'
`__stderr__'
These objects contain the original values of `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.
`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.
`version'
A string containing the version number of the Python interpreter
plus additional information on the build number and compiler used.
It has a value of the form `'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)]'
`version_info'
A tuple containing the five components of the version number:
MAJOR, MINOR, MICRO, RELEASELEVEL, and SERIAL. All values except
RELEASELEVEL are integers; the release level is `'alpha'',
`'beta'', `'candidate'', or `'final''. The `version_info' value
corresponding to the Python version 2.0 is `(2, 0, 0, 'final', 0)'.
_Added in Python version 2.0_
`winver'
The version number used to form registry keys on Windows platforms.
This is stored as string resource 1000 in the Python DLL. The
value is normally the first three characters of `version'. It is
provided in the `sys' module for informational purposes; modifying
this value has no effect on the registry keys used by Python.
Availability: Windows.
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