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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, localscan 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 expressionnormally has full access to the standard builtins module and restricted environments are propagated. If the localsdictionary 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.

　　執(zhí)行動態(tài)標表達式求值

說明：　　

　　1. 執(zhí)行動態(tài)語句，返回語句執(zhí)行的值。

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

　　2. 第一個參數(shù)為語句字符串，globals參數(shù)和locals參數(shù)為可選參數(shù)，如果提供，globals參數(shù)必需是字典，locals參數(shù)為mapping對象。

　　3. globals參數(shù)用來指定代碼執(zhí)行時可以使用的全局變量以及收集代碼執(zhí)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(zhí)行
4

　　4. locals參數(shù)用來指定代碼執(zhí)行時可以使用的局部變量以及收集代碼執(zhí)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(shù)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(nèi)置函數(shù)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(shù)不提供是，Python默認使用globals()函數(shù)返回的字典去調(diào)用。當locals參數(shù)不提供時，默認使用globals參數(shù)去調(diào)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(shù)未提供，locals參數(shù)=globals參數(shù)
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(shù)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)>}
>>>

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