Python 是如何寻找包的?

sys.path

Python import 时会首先寻找 sys.path 中列出的路径,通常是这样:

>>> import sys
>>> '\n'.join(sys.path)

/usr/lib/python2.7
/usr/lib/python2.7/plat-x86_64-linux-gnu
/usr/lib/python2.7/lib-tk
/usr/lib/python2.7/lib-old
/usr/lib/python2.7/lib-dynload
/usr/local/lib/python2.7/dist-packages
/usr/lib/python2.7/dist-packages

那么 Python 是如何获取 sys.path 的呢?根据 Python 的文档,首先是当前目录,然后是 PYTHONPATH 环境变量,再之后是安装时设置的默认目录,由 site 模块控制。

Python 在启动的时候会自动引用 site 模块,关于它是如何操作 sys.path 的,可以参考相关文档

当然,你也可以手动操作 sys.path,这同样会影响会话期间 Python 寻找包的行为。

模块的 __file__ 属性

在引用模块的时候,你可以检查其 __file__ 属性,由此获知它在文件系统的安装位置。

Python 的文档中这样说到:

The __file__ attribute is not present for C modules that are statically linked into the interpreter; for extension modules loaded dynamically from a shared library, it is the pathname of the shared library file.

C 模块并没有 __file__ 属性,因为它们通常是静态链接到解释器的;对于共享库动态加载的扩展模块,只有那个共享库所在的目录名。

例如:

>>> import sys
>>> sys.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute '__file__'

imp 模块

Python 的 import 流程由 imp 模块 控制,因此我们可以完全自己手动控制库的 import 过程。

使用 imp.find_module 来寻找模块:

>>> import imp
>>> imp.find_module('numpy')
(None, '/usr/local/lib/python2.7/dist-packages/numpy', ('', '', 5))

当然,我们可以使用 imp.load_source 直接 import 某个包文件并指定其包名。

imp.load_source('module_name', 'module_full_path')

print(module_name)

Ubuntu 自带的 Python

如果在 Ubuntu 中自己手动编译安装 Python 有时会导致无法找到已安装的包是怎么回事呢?我们来看一下它们的 sys.path

  • Ubuntu Python(/usr/bin/python)
>>> import sys
>>> '\n'.join(sys.path)

/usr/lib/python2.7
/usr/lib/python2.7/plat-x86_64-linux-gnu
/usr/lib/python2.7/lib-tk
/usr/lib/python2.7/lib-old
/usr/lib/python2.7/lib-dynload
/usr/local/lib/python2.7/dist-packages
/usr/lib/python2.7/dist-packages
  • 自己编译的 Python(/usr/local/bin/python)
>>> import sys
>>> '\n'.join(sys.path)

/usr/local/lib/python2.7/dist-packages
/usr/local/lib/python27.zip
/usr/local/lib/python2.7
/usr/local/lib/python2.7/plat-linux2
/usr/local/lib/python2.7/lib-tk
/usr/local/lib/python2.7/lib-old
/usr/local/lib/python2.7/lib-dynload
/usr/local/lib/python2.7/site-packages

Ubuntu Python 会将包安装在 /usr/local/lib/python2.7/dist-packages,而自己编译的 Python 则会安装在 /usr/local/lib/python2.7/site-packages。因此需要手动修改 PYTHONPATH 环境变量来解决。

但是为什么 Ubuntu Python 的 sys.path 中会有 /usr/local/lib/python2.7/dist-packages?实际上它将它硬编码在 site.py 文件中了,首先看一下该文件顶部的注释:

For Debian and derivatives, this sys.path is augmented with directories for packages distributed within the distribution. Local addons go into /usr/local/lib/python/dist-packages, Debian addons install into /usr/{lib,share}/python/dist-packages. /usr/lib/python/site-packages is not used.

对于 Debian 及其衍生系统,sys.path 中是否应该包含包路径是有争议的,本地扩展将安装在 /usr/local/lib/python/dist-packages,Debian 扩展将安装在 /usr/{lib,share}/python/dist-packages,而 /usr/lib/python/site-packages 未被使用。

在来看它具体在代码中是怎样实现的:

def getsitepackages():
    """Returns a list containing all global site-packages directories
    (and possibly site-python).

    For each directory present in the global ``PREFIXES``, this function
    will find its `site-packages` subdirectory depending on the system
    environment, and will return a list of full paths.
    """
    sitepackages = []
    seen = set()

    for prefix in PREFIXES:
        if not prefix or prefix in seen:
            continue
        seen.add(prefix)

        if sys.platform in ('os2emx', 'riscos'):
            sitepackages.append(os.path.join(prefix, "Lib", "site-packages"))
        elif os.sep == '/':
            sitepackages.append(os.path.join(prefix, "local/lib",
                                        "python" + sys.version[:3],
                                        "dist-packages"))
            sitepackages.append(os.path.join(prefix, "lib",
                                        "python" + sys.version[:3],
                                        "dist-packages"))
        else:
            sitepackages.append(prefix)
            sitepackages.append(os.path.join(prefix, "lib", "site-packages"))
        if sys.platform == "darwin":
            # for framework builds *only* we add the standard Apple
            # locations.
            from sysconfig import get_config_var
            framework = get_config_var("PYTHONFRAMEWORK")
            if framework:
                sitepackages.append(
                        os.path.join("/Library", framework,
                            sys.version[:3], "site-packages"))
    return sitepackages

参考:

Kane Blueriver all right reserved,powered by Gitbook最近修改时间: 2018-05-21 17:28:09

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