How to Use eval Function in Python: Syntax and Examples

The eval() function in Python takes a string expression and runs it as Python code, returning the result. Use it by passing a string containing a valid Python expression, like eval('2 + 3'), which returns 5.

Syntax

The eval() function syntax is simple:

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

expression: A string containing a Python expression to evaluate.

globals and locals: Optional dictionaries to specify the global and local variables available during evaluation.

python

result = eval('3 * 4 + 5')
print(result)

Output

17

Example

This example shows how eval() evaluates a string expression and returns the result.

python

expression = '10 + 20 / 5'
result = eval(expression)
print(f"The result of '{expression}' is {result}")

Output

The result of '10 + 20 / 5' is 14.0

Common Pitfalls

Using eval() can be risky because it runs any code given as a string, which can be dangerous if the input is from an untrusted source. Avoid using eval() on user input without strict validation.

Also, eval() only works with expressions, not statements like for loops or if blocks.

python

user_input = 'os.system("rm -rf /")'  # Dangerous!
# Dangerous use:
# eval(user_input)  # NEVER run this on untrusted input

# Safer alternative: use literal_eval for literals only
from ast import literal_eval
safe_input = '[1, 2, 3]'
result = literal_eval(safe_input)
print(result)

Output

[1, 2, 3]

Quick Reference

Tips for using eval() safely and effectively:

• Only use eval() with trusted input.
• Use globals and locals parameters to control the environment.
• For simple data structures, prefer ast.literal_eval() which is safer.
• Remember eval() evaluates expressions, not statements.

Key Takeaways

Use eval() to run Python expressions given as strings and get their results.

Never use eval() on untrusted input to avoid security risks.

Use globals and locals to limit the evaluation environment.

For safe parsing of literals, prefer ast.literal_eval() instead of eval().

eval() works only with expressions, not full Python statements.