import module1,module2
import module as mod
from module import sth
from module import *

import sys;
if not "/home/a/" in sys.path:
    sys.path.append("/home/a/")
if not 'b' in sys.modules:
    b = __import__('b')
else:
    eval('import b')
    b = eval('reload(b)')

# 当前目录:
# now.py 
import pkg.mod1
now = 0
print "In now"

# pkg目录下:(包含空白__init__.py)

# __init__.py
p=-1

# mod1.py
import pkg.mod2
m1=1
print "In mod1"

# mod2.py
m2=2
print "In mod2"

# 以下是交互模式下输入, 输出用->表示

import sys
import now
# -> In mod2 ; In mod1 ; In now
# 可见加载顺序是递归的.

print now.now; 
# -> 0; 
# 需要用命名空间now

print pkg.p; print pkg.mod1.m1; print pkg.mod2.m2
# -> Traceback (most recent call last):
# ->  File "<stdin>", line 1, in <module>
# -> NameError: name 'pkg' is not defined
# pkg的名字存在于now模块中,但不存在当前模块! 更别说子模块了.

sys.modules["now"];
# -> <module 'now' from 'now.py'>
sys.modules["pkg"];
# -> <module 'pkg' from 'pkg/__init__.py'>
sys.modules["pkg.mod1"];
# -> <module 'pkg.mod1' from 'pkg\mod1.py'>
sys.modules["pkg.mod2"];
# -> <module 'pkg.mod2' from 'pkg\mod2.py'>
# 可见四个模块都被加载了,

locals()
# -> {'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, 'sys': <module 'sys' (built-in)>, 'now': <module 'now' from 'now.pyc'>, '__name__': '__main__', '__doc__': None}
# 因为只是加载now,所以里面的子内容都没有加载.

import pkg;locals()
# ->{'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, 'sys': <module 'sys' (built-in)>, 'now': <module 'now' from 'now.pyc'>, 'pkg': <module 'pkg' from 'pkg\__init__.py'>, '__name__': '__main__', '__doc__': None}
# 新加入了pkg, 但没有子模块.

print pkg.p;print pkg.mod1.m1;print pkg.mod2.m2
# -> -1 ; 1 ; 2
# 第一个很容易理解,因为pkg被加载到locals了. 但mod1和mod2没有啊..? 再做个测试就明白了.

# 退出python, 重头再来
import pkg.mod1
# -> In mod2 ; In mod1

print pkg.p;print pkg.mod1.m1;print pkg.mod2.m2
# -> -1 ; 1 ; 2

locals()
# -> {'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, 'sys': <module 'sys' (built-in)>, 'pkg': <module 'pkg' from 'pkg\__init__.pyc'>, '__name__': '__main__', '__doc__': None}

# 由此可见,命名空间中即使我import pkg.mod1, 但命名空间中只有pkg! 也就是说,一旦命名空间加入包名,就可以调用包内已加载的模组内容! 

# 退出python, 重头再来
import sys,pkg.mod2
# -> In mod2

print pkg.p;print pkg.mod2.m2
# -> -1 ; 2
print pkg.mod1.m1;
# -> Traceback (most recent call last):
# ->  File "<stdin>", line 1, in <module>
# -> AttributeError: 'module' object has no attribute 'mod1'
# 虽然pkg被加载到Local,但pkg对象没有mod1属性(模组属性)

sys.modules["pkg.mod2"];
# -> <module 'pkg.mod2' from 'pkg\mod2.pyc'> 变pyc了~
sys.modules["pkg.mod1"]
# -> Traceback (most recent call last):
# ->   File "<stdin>", line 1, in <module>
# -> KeyError: 'pkg.mod1'
# 原因还是因为该模组mod1没有载入,不存在于pkg模组的__dict__当中.

import sys
import numpy as np
sys.modules["numpy"]
# -> <module 'numpy' from 'c:\Python27\lib\site-packages\numpy\__init__.pyc'>
sys.modules["np"]
# -> Traceback (most recent call last):
# ->   File "<stdin>", line 1, in <module>
# -> KeyError: 'np'
# 由此可见, 模组是按原名载入

type(np)
# -> <type 'module'>
type(numpy)
# -> Traceback (most recent call last):
# ->   File "<stdin>", line 1, in <module>
# -> NameError: name 'numpy' is not defined
# 可见, 命名空间只有np而没有numpy
locals()
# -> {'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, 'sys': <module 'sys' (built-in)>, 'np': <module 'numpy' from 'c:\Python27\lib\site-packages\numpy\__init__.pyc'>, '__name__': '__main__', '__doc__': None}
# 结论也是np命名空间存在代替numpy模块

import numpy
locals()
# -> {'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, 'sys': <module 'sys' (built-in)>, 'np': <module 'numpy' from 'c:\Python27\lib\site-packages\numpy\__init__.pyc'>, '__name__': '__main__', 'numpy': <module 'numpy' from 'c:\Python27\lib\site-packages\numpy\__init__.pyc'>, '__doc__': None}
# 再次加载numpy后,numpy并没有实际再次加载创建对象, 但命名空间加入了numpy

# A.py
from B import D
class C:pass

# B.py
from A import C
class D: pass

# Test 1
# 运行:
import A
# -> Traceback (most recent call last):
# ->   File "<stdin>", line 1, in <module>
# ->   File "A.py", line 1, in <module>
# ->     from B import D
# ->   File "B.py", line 1, in <module>
# ->     from A import C
# -> ImportError: cannot import name C

# Test 2
# 外部执行:
python A.py
# -> Traceback (most recent call last):
# ->   File "A.py", line 1, in <module>
# ->     from B import D
# ->   File "c:\Users\Hom\Desktop\B.py", line 1, in <module>
# ->     from A import C
# ->   File "c:\Users\Hom\Desktop\A.py", line 1, in <module>
# ->     from B import D
# -> ImportError: cannot import name D

# Test 3
# 若A.py 中改为import B:
import A
# -> Traceback (most recent call last):
# ->   File "<stdin>", line 1, in <module>
# ->   File "A.py", line 1, in <module>
# ->     import B
# ->   File "B.py", line 1, in <module>
# ->     from A import C
# -> ImportError: cannot import name C

# Test 4
# 外部执行:
python A.py
# ->