Python eval()

Description

eval() 函数用来执行一个字符串表达式，并返回表达式的值。

The syntax of eval() is:

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

Parameters

The eval() function takes three parameters:

• expression - the string parsed and evaluated as a Python expression
• globals (optional) - a dictionary
• locals (optional)- a mapping object. Dictionary is the standard and commonly used mapping type in Python.

• expression -- 表达式。
• 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。
• 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

Return Value

The eval() method returns the result evaluated from the expression.

返回表达式计算结果。

Example 1: How eval() works in Python

x = 1
print(eval('x + 1'))

Output

2

Here, the eval() function evaluates the expression x + 1 and print is used to display this value.

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Example 2: Practical Example to Demonstrate Use of eval()

# Perimeter of Square
def calculatePerimeter(l):
    return 4*l

# Area of Square
def calculateArea(l):
    return l*l


expression = input("Type a function: ")

for l in range(1, 5):
    if (expression == 'calculatePerimeter(l)'):
        print("If length is ", l, ", Perimeter = ", eval(expression))
    elif (expression == 'calculateArea(l)'):
        print("If length is ", l, ", Area = ", eval(expression))
    else:
        print('Wrong Function')
        break

Output

Type a function: calculateArea(l)
If length is  1 , Area =  1
If length is  2 , Area =  4
If length is  3 , Area =  9
If length is  4 , Area =  16