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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>6. 模組 — Python 3.7.0 說明文件</title>

title="在 Python 3.7.0 說明文件中搜尋"

href="../_static/opensearch.xml"/>

</head><body>

<ul>

<a href="../genindex.html" title="General Index"

accesskey="I">索引</a></li>

<a href="../py-modindex.html" title="Python 模組索引"

>模組</a> |</li>

<a href="inputoutput.html" title="7. 輸入和輸出"

accesskey="N">下一頁</a> |</li>

<a href="datastructures.html" title="5. 資料結構"

accesskey="P">上一頁</a> |</li>

<li><img src="../_static/py.png" alt=""

style="vertical-align: middle; margin-top: -1px"/></li>

<li><a href="https://www.python.org/">Python</a> »</li>

<li>

zh_TW

3.7.0

<a href="../index.html">Documentation </a> »

</li>

<li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python 教學</a> »</li>

</form>

</div>

</li>

</ul>

</div>

如果從 Python 直譯器離開後又再次進入，之前（幫函式或變數）做的定義都會消失。因此，想要寫一些比較長的程式時，你最好使用編輯器來準備要輸入給直譯器的內容，並且用該檔案來運行它。這就是一個腳本（script）。隨著你的程式越變越長，你可能會想要把它分開成幾個檔案，讓它比較好維護。你可能也會想用一個你之前已經在其他程式寫好的函式，但不想要複製該函式的原始定義到所有使用它的程式裡。

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

module; definitions from a module can be imported into other modules or into

the main module (the collection of variables that you have access to in a

script executed at the top level and in calculator mode).

A module is a file containing Python definitions and statements. The file name

is the module name with the suffix <code class="file docutils literal notranslate">.py</code> appended. Within a module, the

module’s name (as a string) is available as the value of the global variable

<code class="docutils literal notranslate">__name__</code>. For instance, use your favorite text editor to create a file

called <code class="file docutils literal notranslate">fibo.py</code> in the current directory with the following contents:

<div class="highlight-python3 notranslate"><div class="highlight"><pre># Fibonacci numbers module

def fib(n): # write Fibonacci series up to n

a, b = 0, 1

while a < n:

print(a, end=' ')

a, b = b, a+b

print()

def fib2(n): # return Fibonacci series up to n

result = []

a, b = 0, 1

while a < n:

result.append(a)

return result

</pre></div>

</div>

Now enter the Python interpreter and import this module with the following

command:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import fibo

</pre></div>

</div>

This does not enter the names of the functions defined in <code class="docutils literal notranslate">fibo</code> directly in

the current symbol table; it only enters the module name <code class="docutils literal notranslate">fibo</code> there. Using

the module name you can access the functions:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> fibo.fib(1000)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

>>> fibo.fib2(100)

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

>>> fibo.__name__

'fibo'

</pre></div>

</div>

If you intend to use a function often you can assign it to a local name:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> fib = fibo.fib

>>> fib(500)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

</pre></div>

</div>

<h2>6.1. More on Modules<a class="headerlink" href="#more-on-modules" title="本標題的永久連結">¶</a></h2>

A module can contain executable statements as well as function definitions.

These statements are intended to initialize the module. They are executed only

the first time the module name is encountered in an import statement. <a class="footnote-reference" href="#id2" id="id1">[1]</a>

(They are also run if the file is executed as a script.)

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 variables. 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 functions, <code class="docutils literal notranslate">modname.itemname</code>.

Modules can import other modules. It is customary but not required to place all

<a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> 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 <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statement that imports names from a

module directly into the importing module’s symbol table. For example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> from fibo import fib, fib2

>>> fib(500)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

</pre></div>

</div>

This does not introduce the module name from which the imports are taken in the

local symbol table (so in the example, <code class="docutils literal notranslate">fibo</code> is not defined).

There is even a variant to import all names that a module defines:

>>> fib(500)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

</pre></div>

</div>

This imports all names except those beginning with an underscore (<code class="docutils literal notranslate">_</code>).

In most cases Python programmers do not use this facility since it introduces

an unknown set of names into the interpreter, possibly hiding some things

you have already defined.

Note that in general the practice of importing <code class="docutils literal notranslate">*</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.

If the module name is followed by <a class="reference internal" href="../reference/compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate">as</code></a>, then the name

following <a class="reference internal" href="../reference/compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate">as</code></a> is bound directly to the imported module.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import fibo as fib

>>> fib.fib(500)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

</pre></div>

</div>

This is effectively importing the module in the same way that <code class="docutils literal notranslate">import fibo</code>

will do, with the only difference of it being available as <code class="docutils literal notranslate">fib</code>.

It can also be used when utilising <a class="reference internal" href="../reference/simple_stmts.html#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> with similar effects:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> from fibo import fib as fibonacci

>>> fibonacci(500)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

</pre></div>

</div>

備註

For efficiency reasons, each module is only imported once per interpreter

session. Therefore, if you change your modules, you must restart the

interpreter – or, if it’s just one module you want to test interactively,

use <a class="reference internal" href="../library/importlib.html#importlib.reload" title="importlib.reload"><code class="xref py py-func docutils literal notranslate">importlib.reload()</code></a>, e.g. <code class="docutils literal notranslate">import importlib;

importlib.reload(modulename)</code>.

</div>

<h3>6.1.1. Executing modules as scripts<a class="headerlink" href="#executing-modules-as-scripts" title="本標題的永久連結">¶</a></h3>

When you run a Python module with

<div class="highlight-python3 notranslate"><div class="highlight"><pre>python fibo.py <arguments>

</pre></div>

</div>

the code in the module will be executed, just as if you imported it, but with

the <code class="docutils literal notranslate">__name__</code> set to <code class="docutils literal notranslate">"__main__"</code>. That means that by adding this code at

the end of your module:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>if __name__ == "__main__":

import sys

fib(int(sys.argv[1]))

</pre></div>

</div>

you can make the file usable as a script as well as an importable module,

because the code that parses the command line only runs if the module is

executed as the 「main」 file:

<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python fibo.py 50

0 1 1 2 3 5 8 13 21 34

</pre></div>

</div>

If the module is imported, the code is not run:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import fibo

>>>

</pre></div>

</div>

This is often used either to provide a convenient user interface to a module, or

for testing purposes (running the module as a script executes a test suite).

</div>

<h3>6.1.2. The Module Search Path<a class="headerlink" href="#the-module-search-path" title="本標題的永久連結">¶</a></h3>

When a module named <code class="xref py py-mod docutils literal notranslate">spam</code> is imported, the interpreter first searches for

a built-in module with that name. If not found, it then searches for a file

named <code class="file docutils literal notranslate">spam.py</code> in a list of directories given by the variable

<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate">sys.path</code></a>. <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate">sys.path</code></a> is initialized from these locations:

<li>The directory containing the input script (or the current directory when no

file is specified).</li>

<li><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate">PYTHONPATH</code></a> (a list of directory names, with the same syntax as the

shell variable <code class="xref std std-envvar docutils literal notranslate">PATH</code>).</li>

<li>The installation-dependent default.</li>

</ul>

備註

On file systems which support symlinks, the directory containing the input

script is calculated after the symlink is followed. In other words the

directory containing the symlink is not added to the module search path.

</div>

After initialization, Python programs can modify <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate">sys.path</code></a>. The

directory containing the script being run is placed at the beginning of the

search path, ahead of the standard library path. This means that scripts in that

directory will be loaded instead of modules of the same name in the library

directory. This is an error unless the replacement is intended. See section

<a class="reference internal" href="#tut-standardmodules">Standard Modules</a> for more information.

</div>

<h3>6.1.3. 「Compiled」 Python files<a class="headerlink" href="#compiled-python-files" title="本標題的永久連結">¶</a></h3>

To speed up loading modules, Python caches the compiled version of each module

in the <code class="docutils literal notranslate">__pycache__</code> directory under the name <code class="file docutils literal notranslate">module.version.pyc</code>,

where the version encodes the format of the compiled file; it generally contains

the Python version number. For example, in CPython release 3.3 the compiled

version of spam.py would be cached as <code class="docutils literal notranslate">__pycache__/spam.cpython-33.pyc</code>. This

naming convention allows compiled modules from different releases and different

versions of Python to coexist.

Python checks the modification date of the source against the compiled version

to see if it’s out of date and needs to be recompiled. This is a completely

automatic process. Also, the compiled modules are platform-independent, so the

same library can be shared among systems with different architectures.

Python does not check the cache in two circumstances. First, it always

recompiles and does not store the result for the module that’s loaded directly

from the command line. Second, it does not check the cache if there is no

source module. To support a non-source (compiled only) distribution, the

compiled module must be in the source directory, and there must not be a source

module.

Some tips for experts:

<li>You can use the <a class="reference internal" href="../using/cmdline.html#cmdoption-o"><code class="xref std std-option docutils literal notranslate">-O</code></a> or <a class="reference internal" href="../using/cmdline.html#cmdoption-oo"><code class="xref std std-option docutils literal notranslate">-OO</code></a> switches on the Python command

to reduce the size of a compiled module. The <code class="docutils literal notranslate">-O</code> switch removes assert

statements, the <code class="docutils literal notranslate">-OO</code> switch removes both assert statements and __doc__

strings. Since some programs may rely on having these available, you should

only use this option if you know what you’re doing. 「Optimized」 modules have

an <code class="docutils literal notranslate">opt-</code> tag and are usually smaller. Future releases may

change the effects of optimization.</li>

<li>A program doesn’t run any faster when it is read from a <code class="docutils literal notranslate">.pyc</code>

file than when it is read from a <code class="docutils literal notranslate">.py</code> file; the only thing that’s faster

about <code class="docutils literal notranslate">.pyc</code> files is the speed with which they are loaded.</li>

<li>The module <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal notranslate">compileall</code></a> can create .pyc files for all modules in a

directory.</li>

<li>There is more detail on this process, including a flow chart of the

decisions, in <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3147">PEP 3147</a>.</li>

</ul>

</div>

<h2>6.2. Standard Modules<a class="headerlink" href="#standard-modules" title="本標題的永久連結">¶</a></h2>

Python comes with a library of standard modules, described in a separate

document, the Python Library Reference (「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 <a class="reference internal" href="../library/winreg.html#module-winreg" title="winreg: Routines and objects for manipulating the Windows registry. (Windows)"><code class="xref py py-mod docutils literal notranslate">winreg</code></a> module is only

provided on Windows systems. One particular module deserves some attention:

<a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate">sys</code></a>, which is built into every Python interpreter. The variables

<code class="docutils literal notranslate">sys.ps1</code> and <code class="docutils literal notranslate">sys.ps2</code> define the strings used as primary and secondary

prompts:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import sys

>>> sys.ps1

'>>> '

>>> sys.ps2

'... '

>>> sys.ps1 = 'C> '

C> print('Yuck!')

Yuck!

C>

</pre></div>

</div>

These two variables are only defined if the interpreter is in interactive mode.

The variable <code class="docutils literal notranslate">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="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate">PYTHONPATH</code></a>, or from a built-in default if

<a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate">PYTHONPATH</code></a> is not set. You can modify it using standard list

operations:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import sys

>>> sys.path.append('/ufs/guido/lib/python')

</pre></div>

</div>

<h2>6.3. The <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate">dir()</code></a> Function<a class="headerlink" href="#the-dir-function" title="本標題的永久連結">¶</a></h2>

The built-in function <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate">dir()</code></a> is used to find out which names a module

defines. It returns a sorted list of strings:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import fibo, sys

>>> dir(fibo)

['__name__', 'fib', 'fib2']

>>> dir(sys)

['__displayhook__', '__doc__', '__excepthook__', '__loader__', '__name__',

'__package__', '__stderr__', '__stdin__', '__stdout__',

'_clear_type_cache', '_current_frames', '_debugmallocstats', '_getframe',

'_home', '_mercurial', '_xoptions', 'abiflags', 'api_version', 'argv',

'base_exec_prefix', 'base_prefix', 'builtin_module_names', 'byteorder',

'call_tracing', 'callstats', 'copyright', 'displayhook',

'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix',

'executable', 'exit', 'flags', 'float_info', 'float_repr_style',

'getcheckinterval', 'getdefaultencoding', 'getdlopenflags',

'getfilesystemencoding', 'getobjects', 'getprofile', 'getrecursionlimit',

'getrefcount', 'getsizeof', 'getswitchinterval', 'gettotalrefcount',

'gettrace', 'hash_info', 'hexversion', 'implementation', 'int_info',

'intern', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'path',

'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'ps1',

'setcheckinterval', 'setdlopenflags', 'setprofile', 'setrecursionlimit',

'setswitchinterval', 'settrace', 'stderr', 'stdin', 'stdout',

'thread_info', 'version', 'version_info', 'warnoptions']

</pre></div>

</div>

Without arguments, <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate">dir()</code></a> lists the names you have defined currently:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> a = [1, 2, 3, 4, 5]

>>> import fibo

>>> fib = fibo.fib

>>> dir()

['__builtins__', '__name__', 'a', 'fib', 'fibo', 'sys']

</pre></div>

</div>

Note that it lists all types of names: variables, modules, functions, etc.

<a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate">dir()</code></a> 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

<a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate">builtins</code></a>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import builtins

>>> dir(builtins)

['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',

'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning',

'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError',

'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning',

'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',

'FileExistsError', 'FileNotFoundError', 'FloatingPointError',

'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError',

'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError',

'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError',

'MemoryError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented',

'NotImplementedError', 'OSError', 'OverflowError',

'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError',

'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning',

'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError',

'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError',

'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError',

'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning',

'ValueError', 'Warning', 'ZeroDivisionError', '_', '__build_class__',

'__debug__', '__doc__', '__import__', '__name__', '__package__', 'abs',

'all', 'any', 'ascii', 'bin', 'bool', 'bytearray', 'bytes', 'callable',

'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits',

'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit',

'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr',

'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass',

'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview',

'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property',

'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice',

'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars',

'zip']

</pre></div>

</div>

<h2>6.4. Packages<a class="headerlink" href="#packages" title="本標題的永久連結">¶</a></h2>

Packages are a way of structuring Python’s module namespace by using 「dotted

module names」. For example, the module name <code class="xref py py-mod docutils literal notranslate">A.B</code> designates a submodule

named <code class="docutils literal notranslate">B</code> in a package named <code class="docutils literal notranslate">A</code>. 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 Pillow 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: <code class="file docutils literal notranslate">.wav</code>,

<code class="file docutils literal notranslate">.aiff</code>, <code class="file docutils literal notranslate">.au</code>), 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="highlight-text notranslate"><div class="highlight"><pre>sound/ Top-level package

__init__.py Initialize the sound package

formats/ Subpackage for file format conversions

__init__.py

wavread.py

wavwrite.py

aiffread.py

aiffwrite.py

auread.py

auwrite.py

...

effects/ Subpackage for sound effects

__init__.py

echo.py

surround.py

reverse.py

...

filters/ Subpackage for filters

__init__.py

equalizer.py

vocoder.py

karaoke.py

...

</pre></div>

</div>

When importing the package, Python searches through the directories on

<code class="docutils literal notranslate">sys.path</code> looking for the package subdirectory.

The <code class="file docutils literal notranslate">__init__.py</code> files are required to make Python treat the directories

as containing packages; this is done to prevent directories with a common name,

such as <code class="docutils literal notranslate">string</code>, from unintentionally hiding valid modules that occur later

on the module search path. In the simplest case, <code class="file docutils literal notranslate">__init__.py</code> can just be

an empty file, but it can also execute initialization code for the package or

set the <code class="docutils literal notranslate">__all__</code> variable, described later.

Users of the package can import individual modules from the package, for

example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>import sound.effects.echo

</pre></div>

</div>

This loads the submodule <code class="xref py py-mod docutils literal notranslate">sound.effects.echo</code>. It must be referenced with

its full name.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)

</pre></div>

</div>

An alternative way of importing the submodule is:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>from sound.effects import echo

</pre></div>

</div>

This also loads the submodule <code class="xref py py-mod docutils literal notranslate">echo</code>, and makes it available without its

package prefix, so it can be used as follows:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>echo.echofilter(input, output, delay=0.7, atten=4)

</pre></div>

</div>

Yet another variation is to import the desired function or variable directly:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>from sound.effects.echo import echofilter

</pre></div>

</div>

Again, this loads the submodule <code class="xref py py-mod docutils literal notranslate">echo</code>, but this makes its function

<code class="xref py py-func docutils literal notranslate">echofilter()</code> directly available:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>echofilter(input, output, delay=0.7, atten=4)

</pre></div>

</div>

Note that when using <code class="docutils literal notranslate">from package import item</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 class="docutils literal notranslate">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 <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a>

exception is raised.

Contrarily, when using syntax like <code class="docutils literal notranslate">import item.subitem.subsubitem</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.

<h3>6.4.1. Importing * From a Package<a class="headerlink" href="#importing-from-a-package" title="本標題的永久連結">¶</a></h3>

Now what happens when the user writes <code class="docutils literal notranslate">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. This could take a

long time and importing sub-modules might have unwanted side-effects that should

only happen when the sub-module is explicitly imported.

The only solution is for the package author to provide an explicit index of the

package. The <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statement uses the following convention: if a package’s

<code class="file docutils literal notranslate">__init__.py</code> code defines a list named <code class="docutils literal notranslate">__all__</code>, it is taken to be the

list of module names that should be imported when <code class="docutils literal notranslate">from package 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 <code class="file docutils literal notranslate">sound/effects/__init__.py</code> could contain the following

code:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>__all__ = ["echo", "surround", "reverse"]

</pre></div>

</div>

This would mean that <code class="docutils literal notranslate">from sound.effects import *</code> would import the three

named submodules of the <code class="xref py py-mod docutils literal notranslate">sound</code> package.

If <code class="docutils literal notranslate">__all__</code> is not defined, the statement <code class="docutils literal notranslate">from sound.effects import *</code>

does not import all submodules from the package <code class="xref py py-mod docutils literal notranslate">sound.effects</code> into the

current namespace; it only ensures that the package <code class="xref py py-mod docutils literal notranslate">sound.effects</code> has

been imported (possibly running any initialization code in <code class="file docutils literal notranslate">__init__.py</code>)

and then imports whatever names are defined in the package. This includes any

names defined (and submodules explicitly loaded) by <code class="file docutils literal notranslate">__init__.py</code>. It

also includes any submodules of the package that were explicitly loaded by

previous <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statements. Consider this code:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>import sound.effects.echo

import sound.effects.surround

from sound.effects import *

</pre></div>

</div>

In this example, the <code class="xref py py-mod docutils literal notranslate">echo</code> and <code class="xref py py-mod docutils literal notranslate">surround</code> modules are imported in the

current namespace because they are defined in the <code class="xref py py-mod docutils literal notranslate">sound.effects</code> package

when the <code class="docutils literal notranslate">from...import</code> statement is executed. (This also works when

<code class="docutils literal notranslate">__all__</code> is defined.)

Although certain modules are designed to export only names that follow certain

patterns when you use <code class="docutils literal notranslate">import *</code>, it is still considered bad practice in

production code.

Remember, there is nothing wrong with using <code class="docutils literal notranslate">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.

</div>

<h3>6.4.2. Intra-package References<a class="headerlink" href="#intra-package-references" title="本標題的永久連結">¶</a></h3>

When packages are structured into subpackages (as with the <code class="xref py py-mod docutils literal notranslate">sound</code> package

in the example), you can use absolute imports to refer to submodules of siblings

packages. For example, if the module <code class="xref py py-mod docutils literal notranslate">sound.filters.vocoder</code> needs to use

the <code class="xref py py-mod docutils literal notranslate">echo</code> module in the <code class="xref py py-mod docutils literal notranslate">sound.effects</code> package, it can use <code class="docutils literal notranslate">from

sound.effects import echo</code>.

You can also write relative imports, with the <code class="docutils literal notranslate">from module import name</code> form

of import statement. These imports use leading dots to indicate the current and

parent packages involved in the relative import. From the <code class="xref py py-mod docutils literal notranslate">surround</code>

module for example, you might use:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>from . import echo

from .. import formats

from ..filters import equalizer

</pre></div>

</div>

Note that relative imports are based on the name of the current module. Since

the name of the main module is always <code class="docutils literal notranslate">"__main__"</code>, modules intended for use

as the main module of a Python application must always use absolute imports.

</div>

<h3>6.4.3. Packages in Multiple Directories<a class="headerlink" href="#packages-in-multiple-directories" title="本標題的永久連結">¶</a></h3>

Packages support one more special attribute, <a class="reference internal" href="../reference/import.html#__path__" title="__path__"><code class="xref py py-attr docutils literal notranslate">__path__</code></a>. This is

initialized to be a list containing the name of the directory holding the

package’s <code class="file docutils literal notranslate">__init__.py</code> 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.

註解

<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>In fact function definitions are also 『statements』 that are 『executed』; the

execution of a module-level function definition enters the function name in

the module’s global symbol table.</td></tr>

</tbody>

</table>

</div>

<ul>

<li><a class="reference internal" href="#more-on-modules">6.1. More on Modules</a><ul>

<li><a class="reference internal" href="#executing-modules-as-scripts">6.1.1. Executing modules as scripts</a></li>

<li><a class="reference internal" href="#the-module-search-path">6.1.2. The Module Search Path</a></li>

<li><a class="reference internal" href="#compiled-python-files">6.1.3. 「Compiled」 Python files</a></li>

</ul>

</li>

<li><a class="reference internal" href="#standard-modules">6.2. Standard Modules</a></li>

<li><a class="reference internal" href="#the-dir-function">6.3. The <code class="docutils literal notranslate">dir()</code> Function</a></li>

<li><a class="reference internal" href="#packages">6.4. Packages</a><ul>

<li><a class="reference internal" href="#importing-from-a-package">6.4.1. Importing * From a Package</a></li>

<li><a class="reference internal" href="#intra-package-references">6.4.2. Intra-package References</a></li>

<li><a class="reference internal" href="#packages-in-multiple-directories">6.4.3. Packages in Multiple Directories</a></li>

</ul>

</li>

</ul>

</li>

</ul>

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