substring

A substring is a run of characters that appears inside a larger string, contiguous and in order. In Python, a substring of a str is itself a str, so "Py", "tho", and "Python" all count as substrings of "Python". So does the empty string, which is why "" in text is always True.

Contiguity is what separates a substring from a subsequence. Reading "Pto" out of "Python" keeps the characters in order but skips others in between, so it’s a subsequence rather than a substring.

Click the characters below to pick a run of your own, and watch what happens the moment you skip one:

Interactive diagram — enable JavaScript to view.

You get a substring by slicing the original with a start and stop index, as in text[0:2]. Because strings are immutable, the slice is a brand-new string and the original stays untouched.

To ask whether a substring is present, use the in operator. When you need its position, str.find() returns the lowest index where the substring starts, or -1 when it’s absent, and str.index() raises a ValueError in that case instead. str.count() tallies non-overlapping occurrences, while str.startswith() and str.endswith() check whether the string begins or ends with a given substring.

Example

Say you’re scanning a log line to confirm that it recorded an error and to pull out the request ID it carries:

Python 
>>> line = "2026-09-07 ERROR request_id=7f3a timed out"
>>> "ERROR" in line
True
>>> start = line.index("request_id=") + len("request_id=")
>>> line[start:start + 4]
'7f3a'

The in check answers the yes-or-no question without caring where the match sits. Then .index() reports where "request_id=" begins, and adding its length moves you past the label. Slicing four characters from there hands back '7f3a', a new string that’s a substring of line.

How to Check if a Python String Contains a Substring

Tutorial

How to Check if a Python String Contains a Substring

In this tutorial, you'll learn the best way to check whether a Python string contains a substring. You'll also learn about idiomatic ways to inspect the substring further, match substrings with conditions using regular expressions, and search for substrings in pandas.

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For additional information on related topics, take a look at the following resources:

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By Martin Breuss • Updated Oct. 5, 2026
Python Glossary Share Feedback