Python学习笔记011——内置函数eval()

Posted 2020-10-17

1 描述

 eval()  函数用来执行一个字符串表达式，并返回表达式的值

2 语法

原文

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

expression（sourse） -- 字符串表达式。也即，再写该表达式时不能忘记引号“ ”

　　　　=语句字符串

globals -- 变量作用域，全局命名空间，如果被提供，则必须是一个字典对象。

　　　　= 全局变量，若有，必须是字典。

locals -- 变量作用域，局部命名空间，如果被提供，可以是任何映射对象。 

　　　　=局部变量，若有，必须是字典，若无，则等同于globals

3 返回值

返回表达式expression执行结果

4 练习

4.1 执行动态语句

>>> eval("1+2")
3

print(eval("1+2")) #3

4.2 globals 与locals 省略

官方示例

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

 

也可以时多个参数

x = 1
y = 2
a = eval("x+y")

print("a =",a)

运行：

a = 3

注意：

---

 

a = eval("x+y")

等价于

a = eval("x+y",{"x":1,"y":2},{"x":1,"y":2})

---

4.2 globals和locals省略问题

x = 100
y = 200

ls = {"x":1,"y":2}

c = eval("x+y",ls)

print("c =",c)
print("x = %d,y = %d"%(x,y))

运行

c = 3
x = 100,y = 200

---

 

x = 100
y = 200

ls = {"x":1,"y":2}
gs = {"x":3,"y":4}

a = eval("x+y",ls,gs)
print("a =",a)    #a = 7
print("ls[x] = %d,ls[y] = %d"%(ls["x"],ls["y"]))    #ls[x] = 1,ls[y] = 2
print("gs[x] = %d,gs[y] = %d"%(gs["x"],gs["y"]))    #gs[x] = 3,gs[y] = 4
print("x = %d, y = %d"%(x,y))    #x = 100, y = 200

print("-------------------------------")


b = eval("x+y",ls)
print("b =",b)    #b = 3
print("ls[x] = %d,ls[y] = %d"%(ls["x"],ls["y"]))    #ls[x] = 1,ls[y] = 2
print("gs[x] = %d,gs[y] = %d"%(gs["x"],gs["y"]))    #gs[x] = 3,gs[y] = 4
print("x = %d, y = %d"%(x,y))    #x = 100, y = 200

print("-------------------------------")


c = eval("x+y",ls,None)
print("c =",c)    #c = 3
print("ls[x] = %d,ls[y] = %d"%(ls["x"],ls["y"]))    #ls[x] = 1,ls[y] = 2
print("gs[x] = %d,gs[y] = %d"%(gs["x"],gs["y"]))     #gs[x] = 3,gs[y] = 4
print("x = %d, y = %d"%(x,y))    #x = 100, y = 200

print("-------------------------------")


d = eval("x+y",None,gs)
print("d =",d)    d = 7
print("ls[x] = %d,ls[y] = %d"%(ls["x"],ls["y"]))    #ls[x] = 1,ls[y] = 2
print("gs[x] = %d,gs[y] = %d"%(gs["x"],gs["y"]))    #gs[x] = 3,gs[y] = 4
print("x = %d, y = %d"%(x,y))    #x = 100, y = 200

 运行

a = 7
ls[x] = 1,ls[y] = 2
gs[x] = 3,gs[y] = 4
x = 100, y = 200
-------------------------------
b = 3
ls[x] = 1,ls[y] = 2
gs[x] = 3,gs[y] = 4
x = 100, y = 200
-------------------------------
c = 3
ls[x] = 1,ls[y] = 2
gs[x] = 3,gs[y] = 4
x = 100, y = 200
-------------------------------
d = 7
ls[x] = 1,ls[y] = 2
gs[x] = 3,gs[y] = 4
x = 100, y = 200

 

 4.4关于变量绑定时的报错

 尽管 x 已经定义，但是不是全局变量

>>> {"x":1}
{‘x‘: 1}
>>> eval("x+1")   #   在全部变量中没有定义
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name ‘x‘ is not defined

 

>>> a = {"x":1}
>>> eval("a[‘x‘]+1")  # 这里一定要注意不能内外均是双引号，这样会报错
2

 

>>> a = {"x":1}
>>> eval("x+1",a)  #在a中有x的定义，可以执行
2

 

5 原技术文档

 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.

6 参考

https://www.cnblogs.com/sesshoumaru/p/5995712.html

 

 

6  参考