>>> import sys
# 这个时候os这个module已经被加载了，但是在当前作用域并不可见。 >>> sys.modules['os'] <module 'os' from '/usr/lib/python2.7/os.pyc'> >>>
# 访问os会被提示NameError >>> os Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'os' is not defined >>>
# 将os模块导入到当前作用域 >>> import os >>> os <module 'os' from '/usr/lib/python2.7/os.pyc'> >>>
# 从当前作用域删除os >>> del os >>>
# 但是os模块还存在sys.modules中 >>> sys.modules['os'] <module 'os' from '/usr/lib/python2.7/os.pyc'> >>>
# 此时再访问os模块就会报错 >>> os Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'os' is not defined
sys.modules.keys()中的每个模块确实在python启动的时候被导入了，但是它们不像__builtins__那样直接暴露出来了，它们还隐藏着，需要import把它们加入进来。
再比如，module A引用了module B，当import了module A，实际上module B也被间接加载了，但肯定是看不到的。
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__import__(name[, globals[, locals[, fromlist[, level]]]])
Note
This is an advanced function that is not needed in everyday Python programming, unlike importlib.import_module().
这是一个高级功能，在日常的python编程中并不常用，不像importlib.import_module()。
This function is invoked by the import statement. It can be replaced (by importing the __builtin__ module and assigning to__builtin__.__import__) in order to change __import__() is rare, except in cases where you want to import a module whose name is only known at runtime.
这个函数被import语句调用。如果改变import语句的语义(通过导入__builtin__并且使用__builtin__.__import__) 则可以可以代替__import__。
直接使用__import__是非常少见的，除了你需要在运行时刻才知道需要导入的模块名。
The function imports the module name, import statement.
这个函数导入模块名name，默认情况下使用globals和locals去决定如何在包的上下文中解释模块的name。
fromlist指定了需要在模块name中导入的子模块或者对象。
标准的实现没有使用locals，但是只使用了globals去检测import语句的包的上下文关系。
level specifies whether to use absolute or relative imports. The default is -1 which indicates both absolute and relative imports will be attempted. 0 means only perform absolute imports. __import__().
level参数指定了是使用相对导入还是绝对导入。默认的是-1，说明绝对和相对导入都会去尝试。
0的意思是只做绝对导入。
如果level是正整数，则指明了调用__import__的模块需要搜索的父目录的相对路径的数量
When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name is returned.
当name参数是package.module格式时，顶层的包(第一个点号之前的)会被返回，而不是完整的name名称。
无论如何，当存在fromlist参数时，模块名为name的会被返回。
For example, the statement import spam results in bytecode resembling the following code:
举例，import spam的结果和下面的代码类似：
spam = __import__('spam', globals(), locals(), [], -1)
The statement import spam.ham results in this call:
import spam.ham语句的结果如下：
spam = __import__('spam.ham', globals(), locals(), [], -1)
Note how __import__() returns the toplevel module here because this is the object that is bound to a name by the import statement.
On the other hand, the statement from spam.ham import eggs, sausage as saus results in
从其他方面来说，语句from spam.ham import egges, sausage as saus的结果是：
_temp = __import__('spam.ham', globals(), locals(), ['eggs', 'sausage'], -1) eggs = _temp.eggs saus = _temp.sausage
Here, the spam.ham module is returned from __import__(). From this object, the names to import are retrieved and assigned to their respective names.
If you simply want to import a module (importlib.import_module().
Changed in version 2.5: The level parameter was added.
Changed in version 2.5: Keyword support for parameters was added.
举例来说：
>>> t = __import__('django.http', fromlist=['Cookie']) >>> t <module 'django.http' from '/usr/local/lib/python2.7/dist-packages/django/http/__init__.pyc'> >>> t.Cookie <module 'Cookie' from '/usr/lib/python2.7/Cookie.pyc'>
而这样只返回了顶层的django包：
>>> t = __import__('django.http') >>> t <module 'django' from '/usr/local/lib/python2.7/dist-packages/django/__init__.pyc'>