Remove all non numeric characters python

I would not use RegEx for this. It is a lot slower!

Instead let's just use a simple for loop.

TLDR;

This function will get the job done fast...

def filter_non_digits(string: str) -> str:
    result = ''
    for char in string:
        if char in '1234567890':
            result += char
    return result 

The Explanation

Let's create a very basic benchmark to test a few different methods that have been proposed. I will test three methods...

1. For loop method (my idea).
2. List Comprehension method from Jon Clements' answer.
3. RegEx method from Moradnejad's answer.

# filters.py

import re

# For loop method
def filter_non_digits_for(string: str) -> str:
    result = ''
    for char in string:
        if char in '1234567890':
            result += char
    return result 


# Comprehension method
def filter_non_digits_comp(s: str) -> str:
    return ''.join(ch for ch in s if ch.isdigit())


# RegEx method
def filter_non_digits_re(string: str) -> str:
    return re.sub('[^\d]','', string)

Now that we have an implementation of each way of removing digits, let's benchmark each one.

Here is some very basic and rudimentary benchmark code. However, it will do the trick and give us a good comparison of how each method performs.

# tests.py

import time, platform
from filters import filter_non_digits_re,
                    filter_non_digits_comp,
                    filter_non_digits_for


def benchmark_func(func):
    start = time.time()
    # the "_" in the number just makes it more readable
    for i in range(100_000):
        func('afes098u98sfe')
    end = time.time()
    return (end-start)/100_000


def bench_all():
    print(f'# System ({platform.system()} {platform.machine()})')
    print(f'# Python {platform.python_version()}\n')

    tests = [
        filter_non_digits_re,
        filter_non_digits_comp,
        filter_non_digits_for,
    ]

    for t in tests:
        duration = benchmark_func(t)
        ns = round(duration * 1_000_000_000)
        print(f'{t.__name__.ljust(30)} {str(ns).rjust(6)} ns/op')


if __name__ == "__main__":
    bench_all()

Here is the output from the benchmark code.

# System (Windows AMD64)
# Python 3.9.8

filter_non_digits_re             2920 ns/op
filter_non_digits_comp           1280 ns/op
filter_non_digits_for             660 ns/op

As you can see the filter_non_digits_for() funciton is more than four times faster than using RegEx, and about twice as fast as the comprehension method. Sometimes simple is best.

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