How to Use timeit in Python for Measuring Code Execution Time

Use the timeit module in Python to measure how long a piece of code takes to run by calling timeit.timeit() with the code as a string and the number of repetitions. This helps you check performance by running the code multiple times and getting the average time.

Syntax

The basic syntax of timeit.timeit() is:

• stmt: The code you want to time, as a string.
• setup: Code to run once before timing, usually imports or variable setup, as a string.
• number: How many times to run stmt (default is 1,000,000).

The function returns the total time in seconds to run stmt number times.

python

timeit.timeit(stmt='code_to_test()', setup='from __main__ import code_to_test', number=1000000)

Example

This example measures how long it takes to create a list of squares using a list comprehension versus a for loop.

python

import timeit

code_list_comp = '[x*x for x in range(1000)]'
code_for_loop = '''
result = []
for x in range(1000):
    result.append(x*x)
'''

# Time list comprehension
time_list_comp = timeit.timeit(stmt=code_list_comp, number=1000)

# Time for loop
time_for_loop = timeit.timeit(stmt=code_for_loop, number=1000)

print(f'List comprehension time: {time_list_comp:.5f} seconds')
print(f'For loop time: {time_for_loop:.5f} seconds')

Output

List comprehension time: 0.06000 seconds
For loop time: 0.12000 seconds

Common Pitfalls

Common mistakes when using timeit include:

• Not importing functions or variables used in stmt inside setup, causing NameError.
• Using multi-line code incorrectly without triple quotes.
• Running timeit on code with side effects that affect timing.
• Not specifying number and getting misleading results for very fast or slow code.

Always import or define what your timed code needs in setup and use triple quotes for multi-line statements.

python

import timeit

# Wrong: missing import in setup
# timeit.timeit(stmt='my_func()', number=1000)

# Right: import function in setup
# timeit.timeit(stmt='my_func()', setup='from __main__ import my_func', number=1000)

Quick Reference

Parameter	Description
stmt	Code to time, as a string
setup	Setup code run once before timing, as a string
number	How many times to run stmt (default 1,000,000)
repeat	Number of times to repeat timing (use timeit.repeat)
Returns	Total time in seconds to run stmt number times

Key Takeaways

Use timeit.timeit() to measure execution time of small code snippets accurately.

Always include necessary imports or setup code in the setup parameter.

Run the code multiple times with the number parameter for reliable timing.

Use triple quotes for multi-line code in the stmt parameter.

Avoid side effects in timed code to get consistent results.