Python內置函數(19)——eval

英文文檔：

eval (expression, globals=None, locals=None)

The arguments are a string and optional globals and locals. If provided, globals must be a dictionary. If provided, locals can be any mapping object.

The expression argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using the globals and locals dictionaries as global and local namespace. If the globals dictionary is present and lacks ‘__builtins__’, the current globals are copied into globals before expression is parsed. This means that expression normally has full access to the standard builtins module and restricted environments are propagated. If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment where eval() is called. The return value is the result of the evaluated expression. Syntax errors are reported as exceptions. Example:

>>> x = 1
>>> eval('x+1')
2

This function can also be used to execute arbitrary code objects (such as those created by compile()). In this case pass a code object instead of a string. If the code object has been compiled with 'exec' as the mode argument, eval()‘s return value will be None.

Hints: dynamic execution of statements is supported by the exec() function. The globals() and locals() functions returns the current global and local dictionary, respectively, which may be useful to pass around for use by eval() or exec().

See ast.literal_eval() for a function that can safely evaluate strings with expressions containing only literals.

說明：

　　

　　1. 執行動態語句，返回語句執行的值。

>>> eval('1+2+3+4')
10

　　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}

>>> eval('num + 2') #num未定義
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    eval('num + 2')
  File "<string>", line 1, in <module>
NameError: name 'num' is not defined

>>> eval('num + 2',g) #g中有定義num，可執行
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 = {'num1':2}
>>> l = {'num2':4}
>>> eval('num1+num2',g,l)
6

 　　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 = {}
>>> eval('abs(-1)',g)
1

>>> g = {'__builtins__':{}}
>>> eval('abs(-1)',g) #不能使用內置函數了
Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    eval('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
>>> eval('num+2')
3

>>> globals() #返回字典中含有num的key
{'__doc__': None, 'num': 1, '__package__': None, '__name__': '__main__', '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__builtins__': <module 'builtins' (built-in)>}

>>> eval('num+2',{}) #locals參數未提供，locals參數=globals參數
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    eval('num+2',{})
  File "<string>", line 1, in <module>
NameError: name 'num' is not defined

>>> l = locals() 
>>> eval('num+2',{},l) #locals參數含有num的key，能求值
3

>>> locals()
{'__doc__': None, 'l': {...}, 'num': 1, '__package__': None, '__name__': '__main__', '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__builtins__': <module 'builtins' (built-in)>}
>>>