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<title>6. Modules
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<!--Table of Child-Links-->
<A NAME=
"CHILD_LINKS"><STRONG>Subsections
</STRONG></a>
<LI><A href=
"node8.html#SECTION008100000000000000000">6.1 More on Modules
</a>
<LI><A href=
"node8.html#SECTION008110000000000000000">6.1.1 The Module Search Path
</a>
<LI><A href=
"node8.html#SECTION008120000000000000000">6.1.2 ``Compiled'' Python files
</a>
<LI><A href=
"node8.html#SECTION008200000000000000000">6.2 Standard Modules
</a>
<LI><A href=
"node8.html#SECTION008300000000000000000">6.3 The
<tt class=
"function">dir()
</tt> Function
</a>
<LI><A href=
"node8.html#SECTION008400000000000000000">6.4 Packages
</a>
<LI><A href=
"node8.html#SECTION008410000000000000000">6.4.1 Importing * From a Package
</a>
<LI><A href=
"node8.html#SECTION008420000000000000000">6.4.2 Intra-package References
</a>
<LI><A href=
"node8.html#SECTION008430000000000000000">6.4.3 Packages in Multiple Directories
</a>
<!--End of Table of Child-Links-->
<H1><A NAME=
"SECTION008000000000000000000"></A><A NAME=
"modules"></A>
If you quit from the Python interpreter and enter it again, the
definitions you have made (functions and variables) are lost.
Therefore, if you want to write a somewhat longer program, you are
better off using a text editor to prepare the input for the interpreter
and running it with that file as input instead. This is known as creating a
<em>script
</em>. As your program gets longer, you may want to split it
into several files for easier maintenance. You may also want to use a
handy function that you've written in several programs without copying
its definition into each program.
To support this, Python has a way to put definitions in a file and use
them in a script or in an interactive instance of the interpreter.
Such a file is called a
<em>module
</em>; definitions from a module can be
<em>imported
</em> into other modules or into the
<em>main
</em> module (the
collection of variables that you have access to in a script
executed at the top level
A module is a file containing Python definitions and statements. The
file name is the module name with the suffix
<span class=
"file">.py
</span> appended. Within
a module, the module's name (as a string) is available as the value of
the global variable
<code>__name__
</code>. For instance, use your favorite text
editor to create a file called
<span class=
"file">fibo.py
</span> in the current directory
with the following contents:
<div class=
"verbatim"><pre>
# Fibonacci numbers module
def fib(n): # write Fibonacci series up to n
def fib2(n): # return Fibonacci series up to n
Now enter the Python interpreter and import this module with the
<div class=
"verbatim"><pre>
This does not enter the names of the functions defined in
<code>fibo
</code> directly in the current symbol table; it only enters the module name
Using the module name you can access the functions:
<div class=
"verbatim"><pre>
>>> fibo.fib(
1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
>>> fibo.fib2(
100)
[
1,
1,
2,
3,
5,
8,
13,
21,
34,
55,
89]
>>> fibo.__name__
If you intend to use a function often you can assign it to a local name:
<div class=
"verbatim"><pre>
>>> fib = fibo.fib
1 1 2 3 5 8 13 21 34 55 89 144 233 377
<H1><A NAME=
"SECTION008100000000000000000"></A><A NAME=
"moreModules"></A>
A module can contain executable statements as well as function
These statements are intended to initialize the module.
They are executed only the
<em>first
</em> time the module is imported somewhere.
<A NAME=
"tex2html4"
HREF=
"#foot757"><SUP>6.1</SUP></A>
Each module has its own private symbol table, which is used as the
global symbol table by all functions defined in the module.
Thus, the author of a module can use global variables in the module
without worrying about accidental clashes with a user's global
On the other hand, if you know what you are doing you can touch a
module's global variables with the same notation used to refer to its
<code>modname.itemname
</code>.
Modules can import other modules. It is customary but not required to
place all
<tt class=
"keyword">import
</tt> statements at the beginning of a module (or
script, for that matter). The imported module names are placed in the
importing module's global symbol table.
There is a variant of the
<tt class=
"keyword">import
</tt> statement that imports
names from a module directly into the importing module's symbol
<div class=
"verbatim"><pre>
>>> from fibo import fib, fib2
1 1 2 3 5 8 13 21 34 55 89 144 233 377
This does not introduce the module name from which the imports are taken
in the local symbol table (so in the example,
<code>fibo
</code> is not
There is even a variant to import all names that a module defines:
<div class=
"verbatim"><pre>
>>> from fibo import *
1 1 2 3 5 8 13 21 34 55 89 144 233 377
This imports all names except those beginning with an underscore
<H2><A NAME=
"SECTION008110000000000000000"></A><A NAME=
"searchPath"></A>
6.1.1 The Module Search Path
<a id='l2h-
18' xml:id='l2h-
18'
></a>When a module named
<tt class=
"module">spam
</tt> is imported, the interpreter searches
for a file named
<span class=
"file">spam.py
</span> in the current directory,
and then in the list of directories specified by
the environment variable
<a class=
"envvar" id='l2h-
19' xml:id='l2h-
19'
>PYTHONPATH
</a>. This has the same syntax as
the shell variable
<a class=
"envvar" id='l2h-
20' xml:id='l2h-
20'
>PATH
</a>, that is, a list of
directory names. When
<a class=
"envvar" id='l2h-
21' xml:id='l2h-
21'
>PYTHONPATH
</a> is not set, or when the file
is not found there, the search continues in an installation-dependent
default path; on
<span class=
"Unix">Unix
</span>, this is usually
<span class=
"file">.:/usr/local/lib/python
</span>.
Actually, modules are searched in the list of directories given by the
variable
<code>sys.path
</code> which is initialized from the directory
containing the input script (or the current directory),
<a class=
"envvar" id='l2h-
22' xml:id='l2h-
22'
>PYTHONPATH
</a> and the installation-dependent default. This allows
Python programs that know what they're doing to modify or replace the
module search path. Note that because the directory containing the
script being run is on the search path, it is important that the
script not have the same name as a standard module, or Python will
attempt to load the script as a module when that module is imported.
This will generally be an error. See section
<A HREF=
"#standardModules">6.2</A>,
``Standard Modules,'' for more information.
<H2><A NAME=
"SECTION008120000000000000000">
6.1.2 ``Compiled'' Python files
</A>
As an important speed-up of the start-up time for short programs that
use a lot of standard modules, if a file called
<span class=
"file">spam.pyc
</span> exists
in the directory where
<span class=
"file">spam.py
</span> is found, this is assumed to
contain an already-``byte-compiled'' version of the module
<tt class=
"module">spam
</tt>.
The modification time of the version of
<span class=
"file">spam.py
</span> used to create
<span class=
"file">spam.pyc
</span> is recorded in
<span class=
"file">spam.pyc
</span>, and the
<span class=
"file">.pyc
</span> file is ignored if these don't match.
Normally, you don't need to do anything to create the
<span class=
"file">spam.pyc
</span> file. Whenever
<span class=
"file">spam.py
</span> is successfully
compiled, an attempt is made to write the compiled version to
<span class=
"file">spam.pyc
</span>. It is not an error if this attempt fails; if for any
reason the file is not written completely, the resulting
<span class=
"file">spam.pyc
</span> file will be recognized as invalid and thus ignored
later. The contents of the
<span class=
"file">spam.pyc
</span> file are platform
independent, so a Python module directory can be shared by machines of
<LI>When the Python interpreter is invoked with the
<b class=
"programopt">-O
</b> flag,
optimized code is generated and stored in
<span class=
"file">.pyo
</span> files. The
optimizer currently doesn't help much; it only removes
<tt class=
"keyword">assert
</tt> statements. When
<b class=
"programopt">-O
</b> is used,
<em>all
</em>
bytecode is optimized;
<code>.pyc
</code> files are ignored and
<code>.py
</code>files are compiled to optimized bytecode.
<LI>Passing two
<b class=
"programopt">-O
</b> flags to the Python interpreter
(
<b class=
"programopt">-OO
</b>) will cause the bytecode compiler to perform
optimizations that could in some rare cases result in malfunctioning
programs. Currently only
<code>__doc__
</code> strings are removed from the
bytecode, resulting in more compact
<span class=
"file">.pyo
</span> files. Since some
programs may rely on having these available, you should only use this
option if you know what you're doing.
<LI>A program doesn't run any faster when it is read from a
<span class=
"file">.pyc
</span> or
<span class=
"file">.pyo
</span> file than when it is read from a
<span class=
"file">.py
</span> file; the only
thing that's faster about
<span class=
"file">.pyc
</span> or
<span class=
"file">.pyo
</span> files is the
speed with which they are loaded.
<LI>When a script is run by giving its name on the command line, the
bytecode for the script is never written to a
<span class=
"file">.pyc
</span> or
<span class=
"file">.pyo
</span> file. Thus, the startup time of a script may be reduced
by moving most of its code to a module and having a small bootstrap
script that imports that module. It is also possible to name a
<span class=
"file">.pyc
</span> or
<span class=
"file">.pyo
</span> file directly on the command line.
<LI>It is possible to have a file called
<span class=
"file">spam.pyc
</span> (or
<span class=
"file">spam.pyo
</span> when
<b class=
"programopt">-O
</b> is used) without a file
<span class=
"file">spam.py
</span> for the same module. This can be used to distribute a
library of Python code in a form that is moderately hard to reverse
<LI>The module
<a class=
"ulink" href=
"../lib/module-compileall.html"
><tt class=
"module">compileall
</tt></a> <a id='l2h-
23' xml:id='l2h-
23'
></a> can create
<span class=
"file">.pyc
</span> files (or
<span class=
"file">.pyo
</span> files when
<b class=
"programopt">-O
</b> is used) for all modules in a
<H1><A NAME=
"SECTION008200000000000000000"></A><A NAME=
"standardModules"></A>
Python comes with a library of standard modules, described in a separate
document, the
<em class=
"citetitle"><a title=
"Python Library Reference"
>Python Library Reference
</a></em>
(``Library Reference'' hereafter). Some modules are built into the
interpreter; these provide access to operations that are not part of
the core of the language but are nevertheless built in, either for
efficiency or to provide access to operating system primitives such as
system calls. The set of such modules is a configuration option which
also depends on the underlying platform For example,
the
<tt class=
"module">amoeba
</tt> module is only provided on systems that somehow
support Amoeba primitives. One particular module deserves some
attention:
<a class=
"ulink" href=
"../lib/module-sys.html" ><tt class=
"module">sys
</tt></a><a id='l2h-
24' xml:id='l2h-
24'
></a>, which is built into every
Python interpreter. The variables
<code>sys.ps1
</code> and
<code>sys.ps2
</code> define the strings used as primary and secondary
<div class=
"verbatim"><pre>
>>> sys.ps1 = 'C
> '
These two variables are only defined if the interpreter is in
The variable
<code>sys.path
</code> is a list of strings that determines the
interpreter's search path for modules. It is initialized to a default
path taken from the environment variable
<a class=
"envvar" id='l2h-
25' xml:id='l2h-
25'
>PYTHONPATH
</a>, or from
a built-in default if
<a class=
"envvar" id='l2h-
26' xml:id='l2h-
26'
>PYTHONPATH
</a> is not set. You can modify
it using standard list operations:
<div class=
"verbatim"><pre>
>>> sys.path.append('/ufs/guido/lib/python')
<H1><A NAME=
"SECTION008300000000000000000"></A><A NAME=
"dir"></A>
6.3 The
<tt class=
"function">dir()
</tt> Function
The built-in function
<tt class=
"function">dir()
</tt> is used to find out which names
a module defines. It returns a sorted list of strings:
<div class=
"verbatim"><pre>
>>> import fibo, sys
['__name__', 'fib', 'fib2']
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__',
'__stdin__', '__stdout__', '_getframe', 'api_version', 'argv',
'builtin_module_names', 'byteorder', 'callstats', 'copyright',
'displayhook', 'exc_clear', 'exc_info', 'exc_type', 'excepthook',
'exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getdlopenflags',
'getrecursionlimit', 'getrefcount', 'hexversion', 'maxint', 'maxunicode',
'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache',
'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setdlopenflags',
'setprofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout',
'version', 'version_info', 'warnoptions']
Without arguments,
<tt class=
"function">dir()
</tt> lists the names you have defined
<div class=
"verbatim"><pre>
>>> a = [
1,
2,
3,
4,
5]
>>> fib = fibo.fib
['__builtins__', '__doc__', '__file__', '__name__', 'a', 'fib', 'fibo', 'sys']
Note that it lists all types of names: variables, modules, functions, etc.
<tt class=
"function">dir()
</tt> does not list the names of built-in functions and
variables. If you want a list of those, they are defined in the
standard module
<tt class=
"module">__builtin__
</tt><a id='l2h-
27' xml:id='l2h-
27'
></a>:
<div class=
"verbatim"><pre>
>>> import __builtin__
>>> dir(__builtin__)
['ArithmeticError', 'AssertionError', 'AttributeError', 'DeprecationWarning',
'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',
'FloatingPointError', 'FutureWarning', 'IOError', 'ImportError',
'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt',
'LookupError', 'MemoryError', 'NameError', 'None', 'NotImplemented',
'NotImplementedError', 'OSError', 'OverflowError', 'OverflowWarning',
'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError',
'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError',
'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'True',
'TypeError', 'UnboundLocalError', 'UnicodeDecodeError',
'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError',
'UserWarning', 'ValueError', 'Warning', 'WindowsError',
'ZeroDivisionError', '_', '__debug__', '__doc__', '__import__',
'__name__', 'abs', 'apply', 'basestring', 'bool', 'buffer',
'callable', 'chr', 'classmethod', 'cmp', 'coerce', 'compile',
'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod',
'enumerate', 'eval', 'execfile', 'exit', 'file', 'filter', 'float',
'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex',
'id', 'input', 'int', 'intern', 'isinstance', 'issubclass', 'iter',
'len', 'license', 'list', 'locals', 'long', 'map', 'max', 'min',
'object', 'oct', 'open', 'ord', 'pow', 'property', 'quit', 'range',
'raw_input', 'reduce', 'reload', 'repr', 'reversed', 'round', 'set',
'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super',
'tuple', 'type', 'unichr', 'unicode', 'vars', 'xrange', 'zip']
<H1><A NAME=
"SECTION008400000000000000000"></A><A NAME=
"packages"></A>
Packages are a way of structuring Python's module namespace
by using ``dotted module names''. For example, the module name
<tt class=
"module">A.B
</tt> designates a submodule named
"<tt class="samp
">B</tt>" in a package named
"<tt class="samp
">A</tt>". Just like the use of modules saves the authors of different
modules from having to worry about each other's global variable names,
the use of dotted module names saves the authors of multi-module
packages like NumPy or the Python Imaging Library from having to worry
about each other's module names.
Suppose you want to design a collection of modules (a ``package'') for
the uniform handling of sound files and sound data. There are many
different sound file formats (usually recognized by their extension,
for example:
<span class=
"file">.wav
</span>,
<span class=
"file">.aiff
</span>,
<span class=
"file">.au
</span>), so you may need
to create and maintain a growing collection of modules for the
conversion between the various file formats. There are also many
different operations you might want to perform on sound data (such as
mixing, adding echo, applying an equalizer function, creating an
artificial stereo effect), so in addition you will be writing a
never-ending stream of modules to perform these operations. Here's a
possible structure for your package (expressed in terms of a
hierarchical filesystem):
<div class=
"verbatim"><pre>
__init__.py Initialize the sound package
Formats/ Subpackage for file format conversions
Effects/ Subpackage for sound effects
Filters/ Subpackage for filters
When importing the package, Python searches through the directories
on
<code>sys.path
</code> looking for the package subdirectory.
The
<span class=
"file">__init__.py
</span> files are required to make Python treat the
directories as containing packages; this is done to prevent
directories with a common name, such as
"<tt class="samp
">string</tt>", from
unintentionally hiding valid modules that occur later on the module
search path. In the simplest case,
<span class=
"file">__init__.py
</span> can just be an
empty file, but it can also execute initialization code for the
package or set the
<code>__all__
</code> variable, described later.
Users of the package can import individual modules from the
<div class=
"verbatim"><pre>
import Sound.Effects.echo
This loads the submodule
<tt class=
"module">Sound.Effects.echo
</tt>. It must be referenced
<div class=
"verbatim"><pre>
Sound.Effects.echo.echofilter(input, output, delay=
0.7, atten=
4)
An alternative way of importing the submodule is:
<div class=
"verbatim"><pre>
from Sound.Effects import echo
This also loads the submodule
<tt class=
"module">echo
</tt>, and makes it available without
its package prefix, so it can be used as follows:
<div class=
"verbatim"><pre>
echo.echofilter(input, output, delay=
0.7, atten=
4)
Yet another variation is to import the desired function or variable directly:
<div class=
"verbatim"><pre>
from Sound.Effects.echo import echofilter
Again, this loads the submodule
<tt class=
"module">echo
</tt>, but this makes its function
<tt class=
"function">echofilter()
</tt> directly available:
<div class=
"verbatim"><pre>
echofilter(input, output, delay=
0.7, atten=
4)
Note that when using
<code>from
<var>package
</var> import
<var>item
</var></code>, the
item can be either a submodule (or subpackage) of the package, or some
other name defined in the package, like a function, class or
variable. The
<code>import
</code> statement first tests whether the item is
defined in the package; if not, it assumes it is a module and attempts
to load it. If it fails to find it, an
<tt class=
"exception">ImportError
</tt> exception is raised.
Contrarily, when using syntax like
<code>import
<var>item.subitem.subsubitem
</var></code>, each item except for the last must be
a package; the last item can be a module or a package but can't be a
class or function or variable defined in the previous item.
<H2><A NAME=
"SECTION008410000000000000000"></A><A NAME=
"pkg-import-star"></A>
6.4.1 Importing * From a Package
<a id='l2h-
28' xml:id='l2h-
28'
></a>
Now what happens when the user writes
<code>from Sound.Effects import
*
</code>? Ideally, one would hope that this somehow goes out to the
filesystem, finds which submodules are present in the package, and
imports them all. Unfortunately, this operation does not work very
well on Mac and Windows platforms, where the filesystem does not
always have accurate information about the case of a filename! On
these platforms, there is no guaranteed way to know whether a file
<span class=
"file">ECHO.PY
</span> should be imported as a module
<tt class=
"module">echo
</tt>,
<tt class=
"module">Echo
</tt> or
<tt class=
"module">ECHO
</tt>. (For example, Windows
95 has the
annoying practice of showing all file names with a capitalized first
letter.) The DOS
8+
3 filename restriction adds another interesting
problem for long module names.
The only solution is for the package author to provide an explicit
index of the package. The import statement uses the following
convention: if a package's
<span class=
"file">__init__.py
</span> code defines a list
named
<code>__all__
</code>, it is taken to be the list of module names that
should be imported when
<code>from
<var>package
</var> import *
</code> is
encountered. It is up to the package author to keep this list
up-to-date when a new version of the package is released. Package
authors may also decide not to support it, if they don't see a use for
importing * from their package. For example, the file
<span class=
"file">Sounds/Effects/__init__.py
</span> could contain the following code:
<div class=
"verbatim"><pre>
__all__ = [
"echo",
"surround",
"reverse"]
This would mean that
<code>from Sound.Effects import *
</code> would
import the three named submodules of the
<tt class=
"module">Sound
</tt> package.
If
<code>__all__
</code> is not defined, the statement
<code>from Sound.Effects
import *
</code> does
<em>not
</em> import all submodules from the package
<tt class=
"module">Sound.Effects
</tt> into the current namespace; it only ensures that the
package
<tt class=
"module">Sound.Effects
</tt> has been imported (possibly running any
initialization code in
<span class=
"file">__init__.py
</span>) and then imports whatever names are
defined in the package. This includes any names defined (and
submodules explicitly loaded) by
<span class=
"file">__init__.py
</span>. It also includes any
submodules of the package that were explicitly loaded by previous
import statements. Consider this code:
<div class=
"verbatim"><pre>
import Sound.Effects.echo
import Sound.Effects.surround
from Sound.Effects import *
In this example, the echo and surround modules are imported in the
current namespace because they are defined in the
<tt class=
"module">Sound.Effects
</tt> package when the
<code>from...import
</code> statement
is executed. (This also works when
<code>__all__
</code> is defined.)
Note that in general the practice of importing
<code>*
</code> from a module or
package is frowned upon, since it often causes poorly readable code.
However, it is okay to use it to save typing in interactive sessions,
and certain modules are designed to export only names that follow
Remember, there is nothing wrong with using
<code>from Package
import specific_submodule
</code>! In fact, this is the
recommended notation unless the importing module needs to use
submodules with the same name from different packages.
<H2><A NAME=
"SECTION008420000000000000000">
6.4.2 Intra-package References
</A>
The submodules often need to refer to each other. For example, the
<tt class=
"module">surround
</tt> module might use the
<tt class=
"module">echo
</tt> module. In fact,
are so common that the
<tt class=
"keyword">import
</tt> statement first looks in the
containing package before looking in the standard module search path.
Thus, the surround module can simply use
<code>import echo
</code> or
<code>from echo import echofilter
</code>. If the imported module is not
found in the current package (the package of which the current module
is a submodule), the
<tt class=
"keyword">import
</tt> statement looks for a top-level
module with the given name.
When packages are structured into subpackages (as with the
<tt class=
"module">Sound
</tt> package in the example), there's no shortcut to refer
to submodules of sibling packages - the full name of the subpackage
must be used. For example, if the module
<tt class=
"module">Sound.Filters.vocoder
</tt> needs to use the
<tt class=
"module">echo
</tt> module
in the
<tt class=
"module">Sound.Effects
</tt> package, it can use
<code>from
Sound.Effects import echo
</code>.
<H2><A NAME=
"SECTION008430000000000000000">
6.4.3 Packages in Multiple Directories
</A>
Packages support one more special attribute,
<tt class=
"member">__path__
</tt>. This
is initialized to be a list containing the name of the directory
holding the package's
<span class=
"file">__init__.py
</span> before the code in that file
is executed. This variable can be modified; doing so affects future
searches for modules and subpackages contained in the package.
While this feature is not often needed, it can be used to extend the
set of modules found in a package.
<BR><HR><H4>Footnotes
</H4>
<DT><A NAME=
"foot757">... somewhere.
</A><A
HREF=
"node8.html#tex2html4"><SUP>6.1</SUP></A></DT>
In fact function definitions are also `statements' that are
`executed'; the execution enters the function name in the
module's global symbol table.
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