當執行「from xyz import Abc」時，解釋器做了什麼？

Question

解釋者在這些情況下做了什麼？

1. from xyz import Abc
2. from xyz import *

是否有反正解析完整的文件xyz.py？另一種方法應該更快嗎？還是僅僅是一個可讀性問題，人們更喜歡第二種方法？

Answer 1

它不應該在性能方面有所差異，因爲整個模塊都必須以任何方式處理。

區別在於可讀性和名稱空間的污染。爲了最大限度地減少名稱衝突的機會以及名稱隱藏導致的意外行爲，謹慎的做法是僅導入實際使用的對象。

從Martijn Pieters：

所有頂級名導入到一個模塊，也可以除非__all__參數已設置再次從該模塊 進口。因此，你通常最終會以更項目是 添加到您當地的命名空間比你討價還價每個命令後在命名空間

Answer 2

區別：

>>> from math import * 
>>> len(vars()) 
46 
>>> vars() 
{'pow': <built-in function pow>, 'fsum': <built-in function fsum>, 'cosh': <built-in function cosh>, 'ldexp': <built-in function ldexp>, 'hypot': <built-in function hypot>, 'acosh': <built-in function acosh>, 'tan': <built-in function tan>, 'asin': <built-in function asin>, 'isnan': <built-in function isnan>, 'log': <built-in function log>, 'fabs': <built-in function fabs>, 'floor': <built-in function floor>, 'atanh': <built-in function atanh>, 'sqrt': <built-in function sqrt>, '__package__': None, 'frexp': <built-in function frexp>, 'factorial': <built-in function factorial>, 'degrees': <built-in function degrees>, 'pi': 3.141592653589793, 'log10': <built-in function log10>, '__doc__': None, 'asinh': <built-in function asinh>, 'fmod': <built-in function fmod>, 'atan': <built-in function atan>, '__builtins__': <module '__builtin__' (built-in)>, 'copysign': <built-in function copysign>, 'cos': <built-in function cos>, 'ceil': <built-in function ceil>, 'atan2': <built-in function atan2>, 'isinf': <built-in function isinf>, 'sinh': <built-in function sinh>, '__name__': '__main__', 'trunc': <built-in function trunc>, 'expm1': <built-in function expm1>, 'e': 2.718281828459045, 'tanh': <built-in function tanh>, 'radians': <built-in function radians>, 'sin': <built-in function sin>, 'lgamma': <built-in function lgamma>, 'erf': <built-in function erf>, 'erfc': <built-in function erfc>, 'modf': <built-in function modf>, 'exp': <built-in function exp>, 'acos': <built-in function acos>, 'log1p': <built-in function log1p>, 'gamma': <built-in function gamma>} 


>>> ================================ RESTART ================================ 


>>> from math import log10 
>>> len(vars()) 
5 
>>> vars() 
{'__builtins__': <module '__builtin__' (built-in)>, '__package__': None, '__name__': '__main__', 'log10': <built-in function log10>, '__doc__': None}