`eval()` Function in Python [ English ]

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1. Introduction

In Python, the eval() function is a built-in function used to evaluate and execute a Python expression that is written as a string. It reads the expression, evaluates it, and returns the result of that expression.

The function is useful when a program needs to dynamically execute expressions provided by the user or stored as strings.

However, since eval() executes the given string as Python code, it should be used carefully to avoid security risks, especially when dealing with untrusted input.

2. Syntax

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

Parameters

1. expression A string containing a valid Python expression that needs to be evaluated.

2. globals (optional) A dictionary representing global variables available during evaluation.

3. locals (optional) A dictionary representing local variables available during evaluation.

Return Value

• Returns the result of the evaluated expression.

3. Basic Example

x = eval("5 + 3")
print(x)

Output

8

Explanation

• The string "5 + 3" is passed to eval().
• Python evaluates the expression.
• The result 8 is returned and stored in x.

4. Example with Mathematical Expression

result = eval("10 * 5 + 2")
print(result)

Output

52

Explanation

• The expression "10 * 5 + 2" is evaluated according to normal arithmetic rules.
• The result 52 is returned.

5. Example with User Input

expression = input("Enter a mathematical expression: ")
result = eval(expression)

print("Result:", result)

Example Input

8 * 4 + 3

Output

Result: 35

Explanation

• The user enters a mathematical expression.
• eval() evaluates the expression.
• The computed result is printed.

6. Example Using Variables

a = 10
b = 20

result = eval("a + b")
print(result)

Output

30

Explanation

• eval() can access variables that already exist in the program.
• The expression "a + b" is evaluated using the values of a and b.

7. Example with Lists

numbers = eval("[1, 2, 3, 4, 5]")
print(numbers)
print(type(numbers))

Output

[1, 2, 3, 4, 5]
<class 'list'>

Explanation

• The string "[1, 2, 3, 4, 5]" is evaluated.
• Python converts it into a list object.

8. Example with Dictionary

data = eval("{'name':'Rahul', 'age':21}")
print(data)

Output

{'name': 'Rahul', 'age': 21}

Explanation

• eval() evaluates the string as a dictionary.
• A Python dictionary object is created.

9. Security Consideration

Although eval() is powerful, it can be dangerous if used with untrusted input.

Example of a risky expression:

eval("__import__('os').system('ls')")

This can execute system commands, which may harm the system.

Therefore:

• Avoid using eval() with user input from unknown sources.
• Use safer alternatives when possible.

10. Difference Between eval() and exec()

Example:

eval("5 + 3")   # returns 8
exec("print(5 + 3)")  # prints 8

11. Summary

• eval() evaluates a Python expression given as a string.
• It returns the result of the evaluated expression.
• It can evaluate arithmetic expressions, variables, lists, and dictionaries.
• It should be used carefully because it can execute arbitrary code.