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()
