Python eval()方法用于将表达式字符串解析为python表达式，然后在程序中运行python表达式。
通用的python eval()方法语法为：
eval(expression, globals=None, locals=None)
expression：必需的字符串参数，这是正被解析并作为python表达式执行的表达式字符串。
globals：可选参数是一个字典，用于指定可用于执行的表达式。如果未明确提及，则可以使用标准的内置方法，例如'__builtins__'：None元素。
locals：可选参数，用于指定eval()函数可用的局部变量和方法。
现在，为了更好地了解eval()方法，让我们通过不同的示例来了解它。
str1 = 'Hello'
print(eval('str1 =="Hello"'))
print(eval('str1 + ", Python"'))True Hello, Python
from math import *
for l in range(1,3):
func = input("Enter Math Function to Evaluate:\n")
try:
print(eval(func))
except Exception as ex:
print(ex)
break
print('Done')Enter Math Function to Evaluate: abs(-42) 42 Enter Math Function to Evaluate: max(12, 3, 4 , 2, 17) 17 Done
在了解需要传递给eval()方法的函数之前，让我们获取全局和局部作用域中存在的所有函数和变量的列表。
from math import *
def square_root(n):
return sqrt(n)
# Print current global symbol table as a dictionary
print('List of Global symbols:\n',globals())
#Print current local symbol table as a dictionary
print('\nList of Local symbols:\n',locals())
#print list of names in the current local scope
print('\nList of names in current local scope:\n',dir())List of Global symbols:
{'__name__': '__main__',
'__doc__': None,
'__package__': None,
'__loader__': <class '_frozen_importlib.BuiltinImporter'>,
'__spec__': None, '__annotations__': {},
'__builtins__': <module 'builtins' (built-in)>,
'__file__': 'C:/Python/Python361/eval_example1.py',
'acos': <built-in function acos>,
'acosh': <built-in function acosh>,
'asin': <built-in function asin>,
'asinh': <built-in function asinh>,
'atan': <built-in function atan>,
'atan2': <built-in function atan2>,
'atanh': <built-in function atanh>,
'ceil': <built-in function ceil>,
'copysign': <built-in function copysign>,
'cos': <built-in function cos>,
'cosh': <built-in function cosh>,
'degrees': <built-in function degrees>,
'erf': <built-in function erf>,
'erfc': <built-in function erfc>,
'exp': <built-in function exp>,
'expm1': <built-in function expm1>,
'fabs': <built-in function fabs>,
'factorial': <built-in function factorial>,
'floor': <built-in function floor>,
'fmod': <built-in function fmod>,
'frexp': <built-in function frexp>,
'fsum': <built-in function fsum>,
'gamma': <built-in function gamma>,
'gcd': <built-in function gcd>,
'hypot': <built-in function hypot>,
'isclose': <built-in function isclose>,
'isfinite': <built-in function isfinite>,
'isinf': <built-in function isinf>,
'isnan': <built-in function isnan>,
'ldexp': <built-in function ldexp>,
'lgamma': <built-in function lgamma>,
'log': <built-in function log>,
'log1p': <built-in function log1p>,
'log10': <built-in function log10>,
'log2': <built-in function log2>,
'modf': <built-in function modf>,
'pow': <built-in function pow>,
'radians': <built-in function radians>,
'sin': <built-in function sin>,
'sinh': <built-in function sinh>,
'sqrt': <built-in function sqrt>,
'tan': <built-in function tan>,
'tanh': <built-in function tanh>,
'trunc': <built-in function trunc>,
'pi': 3.141592653589793,
'e': 2.718281828459045,
'tau': 6.283185307179586,
'inf': inf,
'nan': nan,
'square_root': <function square_root at 0x055246F0>
}
List of Local symbols:
{'__name__': '__main__', '__doc__': None,
'__package__': None,
'__loader__': <class '_frozen_importlib.BuiltinImporter'>,
'__spec__': None, '__annotations__': {},
'__builtins__': <module 'builtins' (built-in)>,
'__file__': 'C:/Python/Python361/eval_example1.py',
'acos': <built-in function acos>,
'acosh': <built-in function acosh>,
'asin': <built-in function asin>,
'asinh': <built-in function asinh>,
'atan': <built-in function atan>,
'atan2': <built-in function atan2>,
'atanh': <built-in function atanh>,
'ceil': <built-in function ceil>,
'copysign': <built-in function copysign>,
'cos': <built-in function cos>,
'cosh': <built-in function cosh>,
'degrees': <built-in function degrees>,
'erf': <built-in function erf>,
'erfc': <built-in function erfc>,
'exp': <built-in function exp>,
'expm1': <built-in function expm1>,
'fabs': <built-in function fabs>,
'factorial': <built-in function factorial>,
'floor': <built-in function floor>,
'fmod': <built-in function fmod>,
'frexp': <built-in function frexp>,
'fsum': <built-in function fsum>,
'gamma': <built-in function gamma>,
'gcd': <built-in function gcd>,
'hypot': <built-in function hypot>,
'isclose': <built-in function isclose>,
'isfinite': <built-in function isfinite>,
'isinf': <built-in function isinf>,
'isnan': <built-in function isnan>,
'ldexp': <built-in function ldexp>,
'lgamma': <built-in function lgamma>,
'log': <built-in function log>,
'log1p': <built-in function log1p>,
'log10': <built-in function log10>,
'log2': <built-in function log2>,
'modf': <built-in function modf>,
'pow': <built-in function pow>,
'radians': <built-in function radians>,
'sin': <built-in function sin>,
'sinh': <built-in function sinh>,
'sqrt': <built-in function sqrt>,
'tan': <built-in function tan>,
'tanh': <built-in function tanh>,
'trunc': <built-in function trunc>,
'pi': 3.141592653589793,
'e': 2.718281828459045,
'tau': 6.283185307179586,
'inf': inf,
'nan': nan,
'square_root': <function square_root at 0x055246F0>
}
List of names in current local scope:
['__annotations__',
'__builtins__',
'__doc__',
'__file__',
'__loader__',
'__name__',
'__package__',
'__spec__',
'acos', 'acosh',
'asin', 'asinh',
'atan', 'atan2', 'atanh',
'ceil', 'copysign', 'cos',
'cosh', 'degrees', 'e',
'erf', 'erfc', 'exp',
'expm1', 'fabs',
'factorial', 'floor',
'fmod', 'frexp', 'fsum',
'gamma', 'gcd', 'hypot',
'inf', 'isclose', 'isfinite',
'isinf', 'isnan', 'ldexp',
'lgamma', 'log', 'log10',
'log1p', 'log2', 'modf', 'nan',
'pi', 'pow', 'radians', 'sin',
'sinh', 'sqrt', 'square_root',
'tan', 'tanh', 'tau', 'trunc'
]在下面我们可以看到，我们可以在eval函数中将globals值指定为空字典：
>>> print(eval('dir()', {}))
['__builtins__']让我们看另一个示例，在该示例中，我们仅允许访问math模块中的一些方法。
from math import *
for l in range(1, 4):
func = input("Enter Math Function to Evaluate.\nAllowed Functions are: abs(x) and pow(x,y):\n")
try:
print(eval(func, {'absolute': abs, 'pow': pow}))
except Exception as ex:
print(ex)
break
print('Done')输出结果
Enter Math Function to Evaluate. Allowed Functions are: abs(x) and pow(x,y): abs(-74) 74 Enter Math Function to Evaluate. Allowed Functions are: abs(x) and pow(x,y): pow(2,7) 128.0 Enter Math Function to Evaluate. Allowed Functions are: abs(x) and pow(x,y): sqrt(47) name 'sqrt' is not defined Done