Python Timeit, Repeat Examples

This Python article uses the timeit module to benchmark code. It provides examples timeit, repeat, multiple statements and methods.
Timeit. Micro-benchmarks are not always useful. But they are sometimes fun. Which code unit executes faster in a Python program? We answer this question with timeit, a module that benchmarks code fragments.
Example. We introduce timeit with a simple example. We must import timeit with the "import timeit" statement. This is required unless you use the command-line syntax. Here we time two string-creation expressions.

And: We pass the statements in quoted strings to the timeit.timeit method. We increase the iterations by specifying a number argument.

Note: The numbers here are too close to know for sure which is faster. We can use the repeat() method to receive better information.

Python program that uses timeit import timeit # The instructions being timed. print('y' * 3) print('y' + 'y' + 'y') # Call timeit on the statements and print the time returned. # ... Specify optional number of iterations. print(timeit.timeit("x = 'y' * 3", number=10000000)) print(timeit.timeit("x = 'y' + 'y' + 'y'", number=10000000)) Output yyy yyy 0.2625868763293428 0.26622904456542135
Repeat. Repeat is the same as timeit except it benchmarks repeatedly: it calls timeit internally several times. The default repetition is 3. We can increase or decrease this by specifying the repeat argument.
Here, we increase the number of iterations of the string-multiplying code shown in the previous example. We start to get repeatable data. It seems to indicate that adding three strings together is faster than multiplying one by 3.

However: In further testing, I reordered the calls to timeit.repeat so that the multiplication comes second.

And: The program seems to indicate the first call to timeit.repeat is slower. So we learn little about string performance.

Python program that uses repeat import timeit # Call repeat. print(timeit.repeat("x = 'y' * 3", number=100000000, repeat=3)) print(timeit.repeat("x = 'y' + 'y' + 'y'", number=100000000, repeat=3)) Output [2.7390200865497176, 2.7475431168207223, 2.7429300279022177] [2.6369100279087014, 2.631240758828813, 2.6300020650299665]
Command-line. Some programmers extensively use the command-line. I do not. But the timeit module can be invoked directly from the command-line. This avoids creating an entire new program file. Timeit returns usec (microseconds) in the output.

Tip: You will need to be careful with quotation marks when using the command-line. You may need to escape them, depending on your system.

Command line for timeit: Windows 10 C:\Users\Sam>C:\Python33\python.exe -m timeit "x = \"y\" * 3" 10000000 loops, best of 3: 0.0273 usec per loop
Multiple statements. With timeit, we can use multiple statements. We separate them with a semicolon. This is not typical of Python syntax, but it works. This makes it easier to specify longer (but not really long) code fragments.

Note: For longer code fragments, please use the setup argument and call a method. The next example demonstrates.

Python program that uses timeit, semicolon import timeit # Use semicolon for multiple statements. print(timeit.repeat("a = 2; a *= 2", number=100000000)) print(timeit.repeat("a = 1; a *= 4", number=100000000)) Output [7.334341642836696, 7.333336683198793, 7.332224095625474] [7.235993375046725, 7.247406798908553, 7.256258872415835]
Methods, setup. Let us continue with methods. We can benchmark custom methods in timeit by specifying a setup argument. In this argument, please specify an import statement that indicates the methods you invoke.

Here: We benchmark the a() method against the b() method. As expected, the a() method is faster. It does less.

Python program that uses timeit, methods, set up import timeit def a(): return 1 def b(): return sum([-1, 0, 1, 1]) # Test methods. print(a()) print(b()) # Pass setup argument to call methods. print(timeit.repeat("a()", setup="from __main__ import a")) print(timeit.repeat("b()", setup="from __main__ import b")) Output 1 1 [0.11886792269331777, 0.11894442929800975, 0.11940800745355873] [0.5983422704501993, 0.6003713163771788, 0.6014057764431624]
Discussion. Benchmarks are fraught with problems. It is hard, if not impossible, to isolate external factors. And often we should not want to: no programs run without external interference in the real world.
In testing timeit, I found it too has many problems. Its syntax is clunky. After all, what sort of Python program separates statements with semicolons? And the syntax for invoking methods is cumbersome.

Further: I found that the ordering of calls to timeit impacts the results. This makes it harder to trust the results of timeit.

In many ways, I like timeit. It provides a standardized way to perform these benchmarks. And all the alternatives too have problems. Overall, focusing on higher-level concerns and code quality is a better use of your time.Timeit: Python.org

And: With the new JIT compilation technologies, such as PyPy, micro-benchmarks must be updated. Performance vastly changes.

JIT Tests
Summary. It usually costs more time running micro-benchmarks that you will get back in increased speed. But these benchmarks are helpful for learning the performance characteristics of a language. With timeit, we have another benchmarking option.
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