Example of Expression Action of eval Function in Python

What does the directory eval do? The simplest expression in grammar format is chestnut belt globals belt locals string transfer dictionary belt globals belt locals built-in function chestnut error report chestnut

What does eval do?

Parses a string expression and executes, returning 1 value

Grammatical format

eval(expression[, globals[, locals]]) 

expression : Expression string

globals : Must be a dictionary

locals Can be any map object

The simplest expression chestnut

Chestnut 1

print(eval("123"))
print(eval("True"))
print(eval("(1,2,3)"))
print(eval("[1,2,3]")) 
#  Output result 
123
True
(1, 2, 3)
[1, 2, 3]

Chestnut 2

print(eval("1+2")) 
x = 1
print(eval('x+1'))
#  Output result 
3
2

Chestnut 3

a = 1
b = 2
print(eval("[a,b]")) 
#  Output result 
[1, 2]

Take globals with you

#  Use  globals
x = 10
g = {"x": 5}
print(eval("x+1", g))
#  Output result 
6 

The globals parameters are provided in eval

The scope of eval is the dictionary specified by g. The outside x = 10 is masked, and eval is invisible, so the value of x is 5 is used

x = 10
y = 5
g = {"x": 5}
print(eval("x+1+y", g)) 
#  Output result 
5
    print(eval("x+1+y", g))
  File "<string>", line 1, in <module>
NameError: name 'y' is not defined

An error is reported because the global parameter has no y variable value

Take locals with you

#  Use  locals
a = 1
g = {"a": 2, "b": 3}
l = {"b": 30, "c": 4} 
print(eval("a+b+c", g, l))
#  Output result 
36

The scope of eval becomes globals + locals The scope priority of locals will be higher than that of globals The value in the locals parameter overrides the value in the globals parameter

String-to-dictionary

#  String-to-dictionary 
jsons = "{'a':123,'b':True}"
print(type(eval(jsons))) 
#  Output result 
<class 'dict'>

Take globals with you

print(eval("{'name':'linux','age':age}", {"age": 123}))
#  Output result 
{'name': 'linux', 'age': 123}

Take locals with you

print(eval("{'name':'linux','age':age}", {"age": 123}, {"age": 24})) 
#  Output result 
{'name': 'linux', 'age': 24}

Built-in function chestnut

print(eval("123"))
print(eval("True"))
print(eval("(1,2,3)"))
print(eval("[1,2,3]")) 
#  Output result 
123
True
(1, 2, 3)
[1, 2, 3]

0

Chestnuts that report errors

Chestnut 1

print(eval("123"))
print(eval("True"))
print(eval("(1,2,3)"))
print(eval("[1,2,3]")) 
#  Output result 
123
True
(1, 2, 3)
[1, 2, 3]

1

Chestnut 2

print(eval("123"))
print(eval("True"))
print(eval("(1,2,3)"))
print(eval("[1,2,3]")) 
#  Output result 
123
True
(1, 2, 3)
[1, 2, 3]

2

Chestnut 3

print(eval("if x: print(x)"))
#  Output result 
    print(eval("if x: print(x)"))
  File "<string>", line 1
    if x: print(x)
    ^
SyntaxError: invalid syntax

Because eval () only accepts expressions, any other statements (such as if, for, while, import, def, class) will throw an error

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