Python编程：eval函数的使用

eval(expression, [globals=None{dict}, locals=None{dict}])
执行一个字符串表达式

python是用命名空间来记录变量的轨迹的，命名空间是一个dictionary，键是变量名，值是变量值

查找顺序：LEGB

local 局部 locals()
Enclose 闭包
Global 全局  globals()
Build-in 内置

# -*- coding: utf-8 -*-

# @File    : eval_demo.py
# @Date    : 2018-07-13
# @Author  : Peng Shiyu

x = 1
y = 2

ret1 = eval("x+y")
print("global: %s" % ret1)  # global: 3


def func():
    x = 3
    y = 4
    ret2 = eval("x+y")
    print("func: %s" % ret2)  # func: 7

    ret3 = eval("x+y", globals())
    print("func globals: %s" % ret3)  # func globals: 3

    print(locals())


func()
print(globals())
"""
{'y': 4, 'x': 3, 'ret3': 3, 'ret2': 7}

{'__name__': '__main__', '__doc__': 'xxx', 
'__package__': None, '__spec__': None,
'__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x1007dce10>, 
'__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 
'__file__': 'demo/eval_demo.py', '__cached__': None, 
'x': 1, 'y': 2, 'ret1': 3, 'func': <function func at 0x1007d46a8>}

"""

# locals()对象的值不能修改，globals()对象的值可以修改


def func_b():
    x = 20
    print("locals.x %s" % locals()["x"])
    locals()["x"] = 30  # 尝试修改局部变量
    print("locals.x %s" % locals()["x"])

    print("globals.y %s" % globals()["y"])
    globals()["y"] = 40  # 尝试修改全局变量
    print("globals.y %s" % globals()["y"])


func_b()

"""
locals.x 20
locals.x 20
globals.y 2
globals.y 40
"""

参考：
python eval()