[Tut] How to Find a Partial String in a Python List?

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How to Find a Partial String in a Python List?

5/5 – (1 vote)

Problem Formulation

Challenge: Given a Python list of strings and a query string. Find the strings that partially match the query string.

Example 1:

• Input:
  Code:

  ['hello', 'world', 'python']
  and
  Code:

  'pyth'
  
  
• Output:
  Code:

  ['python']

Example 2:

• Input:
  Code:

  ['aaa', 'aa', 'a']
  and
  Code:

  'a'
  
  
• Output:
  Code:

  ['aaa', 'aa', 'a']

Example 3:

• Input:
  Code:

  ['aaa', 'aa', 'a']
  and
  Code:

  'b'
  
  
• Output:
  Code:

  []

Let’s dive into several methods that solve this and similar type of problems. We start with the most straightforward solution.

Method 1: Membership + List Comprehension

The most Pythonic way to find a list of partial matches of a given string

Code:

query

in a string list

Code:

lst

is to use the membership operator

Code:

in

and the list comprehension statement like so:

Code:

[s for s in lst if query in s]

.

Here’s a simple example:

Code:

def partial(lst, query): return [s for s in lst if query in s] # Example 1: print(partial(['hello', 'world', 'python'], 'pyth')) # ['python'] # Example 2: print(partial(['aaa', 'aa', 'a'], 'a')) # ['aaa', 'aa', 'a'] # Example 3: print(partial(['aaa', 'aa', 'a'], 'b')) # []

In case you need some background information, feel free to check out our two tutorials and the referenced videos.

Recommended Tutorial: List Comprehension in Python



Recommended Tutorial: The Membership Operator in Python



Method 2: list() and filter()

To find a list of partial query matches given a string list

Code:

lst

, combine the membership operator with the

Code:

filter()

function in which you pass a lambda function that evaluates the membership operation for each element in the list like so:

Code:

list(filter(lambda x: query in x, lst))

.

Here’s an example:

Code:

def partial(lst, query): return list(filter(lambda x: query in x, lst)) # Example 1: print(partial(['hello', 'world', 'python'], 'pyth')) # ['python'] # Example 2: print(partial(['aaa', 'aa', 'a'], 'a')) # ['aaa', 'aa', 'a'] # Example 3: print(partial(['aaa', 'aa', 'a'], 'b')) # []

Beautiful Python one-liner, isn’t it?

I recommend you check out the following tutorial with video to shed some light on the background information here:

Recommended Tutorial: Python Filtering



Generally, I like list comprehension more than the

Code:

filter()

function because the former is more concise (e.g., no need to convert the result to a list) and slightly faster. But both work perfectly fine!

Method 3: Regex Match + List Comprehension

The most flexible way to find a list of partial query matches given a string list

Code:

lst

is provided by Python’s powerful regular expressions functionality. For example, the expression

Code:

[x for x in lst if re.match(pattern, x)]

finds all strings that match a certain query pattern as defined by you.

The following examples showcase this solution:

Code:

import re def partial(lst, query): pattern = '.*' + query + '.*' return [x for x in lst if re.match(pattern, x)] # Example 1: print(partial(['hello', 'world', 'python'], 'pyth')) # ['python'] # Example 2: print(partial(['aaa', 'aa', 'a'], 'a')) # ['aaa', 'aa', 'a'] # Example 3: print(partial(['aaa', 'aa', 'a'], 'b')) # []

In this example, we use the dummy pattern

Code:

.*query.*

that simply matches words that contain the

Code:

query

string. However, you could also do more advanced pattern matching—regex to the rescue!

Again, I’d recommend you check out the background info on regular expressions:

Recommended Tutorial: Python Regex

Code:

match()

— A Simple Illustrated Guide



Regex Humor

[i]Wait, forgot to escape a space. Wheeeeee[taptaptap]eeeeee. (source)[/i]