Python內置函數(20)——exec

英文文檔：

exec (object[, globals[, locals]])

This function supports dynamic execution of Python code. object must be either a string or a code object. If it is a string, the string is parsed as a suite of Python statements which is then executed (unless a syntax error occurs). [1] If it is a code object, it is simply executed. In all cases, the code that’s executed is expected to be valid as file input (see the section “File input” in the Reference Manual). Be aware that the return and yield statements may not be used outside of function definitions even within the context of code passed to the exec() function. The return value is None.

In all cases, if the optional parts are omitted, the code is executed in the current scope. If only globals is provided, it must be a dictionary, which will be used for both the global and the local variables. If globals and locals are given, they are used for the global and local variables, respectively. If provided, locals can be any mapping object. Remember that at module level, globals and locals are the same dictionary. If exec gets two separate objects as globals and locals, the code will be executed as if it were embedded in a class definition.

If the globals dictionary does not contain a value for the key __builtins__, a reference to the dictionary of the built-in module builtins is inserted under that key. That way you can control what builtins are available to the executed code by inserting your own __builtins__ dictionary into globals before passing it to exec().

Note

The built-in functions globals() and locals() return the current global and local dictionary, respectively, which may be useful to pass around for use as the second and third argument to exec().

Note

The default locals act as described for function locals() below: modifications to the default locals dictionary should not be attempted. Pass an explicit locals dictionary if you need to see effects of the code on locals after function exec() returns.

說明：

　　

　　1.  exec函數和eval函數類似，也是執行動態語句，只不過eval函數只用於執行表達式求值，而exec函數主要用於執行語句塊。

>>> eval('a=1+2') #執行語句報錯
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    eval('a=1+2')
  File "<string>", line 1
    a=1+2
     ^
SyntaxError: invalid syntax

>>> exec('a=1+2') #執行語句
>>> a
3

　　2. 第一個參數為語句字符串，globals參數和locals參數為可選參數，如果提供，globals參數必需是字典，locals參數為mapping對象。

　　3. globals參數用來指定代碼執行時可以使用的全局變量以及收集代碼執行后的全局變量

>>> g = {'num':2}
>>> type(g)
<class 'dict'>

>>> exec('num2 = num + 2',g)

>>> g['num']
2
>>> g['num2'] #收集了exec中定義的num2全局變量
4

 

　　4. locals參數用來指定代碼執行時可以使用的局部變量以及收集代碼執行后的局部變量

>>> g = {'num':2}
>>> type(g)
<class 'dict'>
>>> l = {'num2':3}
>>> type(l)
<class 'dict'>

>>> exec('''
num2 = 13
num3 = num + num2
''',g,l)

>>> l['num2'] #l中num2值已經改變
13

 

　　5. 為了保證代碼成功運行，globals參數字典不包含 __builtins__ 這個 key 時，Python會自動添加一個key為 __builtins__ ，value為builtins模塊的引用。如果確實要限制代碼不使用builtins模塊，需要在global添加一個key為__builtins__，value為{}的項即可（很少有人這么干吧）。

>>> g = {}
>>> exec('a = abs(-1)',g)
>>> 

>>> g = {'__builtins__':{}}
>>> exec('a = abs(-1)',g) #不能使用內置函數了
Traceback (most recent call last):
  File "<pyshell#30>", line 1, in <module>
    exec('a = abs(-1)',g)
  File "<string>", line 1, in <module>
NameError: name 'abs' is not defined

 

　　6. 當globals參數不提供是，Python默認使用globals()函數返回的字典去調用。當locals參數不提供時，默認使用globals參數去調用。

>>> num = 1
>>> exec('num2 = num + 1')
>>> globals()
{'__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__name__': '__main__', '__spec__': None, '__builtins__': <module 'builtins' (built-in)>, '__doc__': None, 'num2': 2, 'num': 1}
>>> 
>>> 
>>> exec('num2 = num + 1',{}) #指定了globals參數，globals中無num變量 執行失敗
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    exec('num2 = num + 1',{})
  File "<string>", line 1, in <module>
NameError: name 'num' is not defined


>>> l = locals()
>>> l
{'__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__name__': '__main__', '__spec__': None, '__builtins__': <module 'builtins' (built-in)>, '__doc__': None, 'l': {...}, 'num2': 2, 'num': 1}
>>> 
>>> exec('num3 = num + 1',{},l)#指定了globals參數，globals中無num變量，指定了locals變量，locals變量含有num變量 執行成功
>>> l
{'__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__name__': '__main__', '__spec__': None, 'num3': 2, '__builtins__': <module 'builtins' (built-in)>, '__doc__': None, 'l': {...}, 'num2': 2, 'num': 1}
>>>