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Substrings in Python
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Substrings in Python

Published On: December 23, 2021

Program slicing and importance of slicing substrings from strings in Python 

Program slicing is an important method for assessing a program. Its role mostly comes during debugging, program understanding, testing and reengineering. If the programmer does not find any value in a part, he/she can remove those parts through program slicing. The slicing procedure begins from a subset and then trims to a reduced form thereby still giving that same behavior of the program. There are various variations of slicing including dynamic slicing, static slicing, quasi static slicing, amorphous slicing etc.

However, static slicing and dynamic slicing are the commonly used types of slicing. The deletion of  a statement is also a form of slicing. To be precise, the reduced program is called the slice. The developer needn’t worry about the complicated codes and can only concentrate on the essential components of the program. Slicing is used in powerful languages like Python

What is slicing in Python?

It is more than  three decades that the concept of slicing has emerged, thanks to Mark Weiser. He observed that developers attributed the root cause of a failure by tracking sequences of statements from where the failure is noted. The key is finding the root cause of failure. With regard to debugging, dynamic slicing has been the method preferred by programmers. The dynamic slicing algorithms can vastly decrease the effort put on debugging by directing the concentration of the user on a subset of selected variables. 

The purpose of slicing in Python is to eliminate complicated codes and write clear and readable codes. One can access parts of sequences including strings, lists, and tuples through slicing. There is also another important aspect called indexing in Python, and one may tend to confuse between indexing and slicing. One should understand that slicing is a good extension of indexing. 

  • While indexing is used to get individual elements, slicing is used to get one or more elements.  
  • While indexing supports both positive and negative indexing, slicing has an equivalent range function that will help you make out which will be considered during the slicing operation. 
  • Slicing out of range of a sequence will not lead to an error. This is because slicing returns a subsequence of items. 

Now, let’s discuss how to obtain substrings by slicing Python strings. Strings are essentially a sequence of characters. String literals can come within either double or single quotes. But note that single quotes are generally used. 

With the help of Python string slicing, you can create a substring. Let’s see the important points here:

  • Python strings are immutable. They cannot be changed once they are created. 
  • To be more elaborate, you can’t change a string’s value. You can only change the reference to the string.
  • Though you may develop new substrings from the source string, the original string doesn’t have any impact. 

Python offers a good way to substring a string through slicing. This adheres to the following template:

string [ start: end: step]

Their respective defaults when not given are None. However, we can pass them explicitly. 

The slicing begins with the start_pos index and ends at end_pos index. Step refers to the steps taken from the beginning to end index. While the start is always included, the end is always excluded. 

P e a c e

0  1  2  3 4

-5 -4 -3 -2 -1

s[1:4] is ‘eac’ — chars beginning at index 1 and extending up to but not including index 4

s[1:] is ‘eace’ – not including either index defaults to the beginning or end of the string

s[:] is ‘Peace’ – not including both always provides us a copy of the entire thing 

s[1:100] is ‘eace’ — an index that is very big is truncated down to the string length

Negative index numbers count back from the last point of the string. 

s[-1] is ‘e’ — last char (1st from the last)

s[-4] is ‘e’ — 4th from the last

s[:-3] is ‘Pe’ — traveling up to but not including the end 3 chars.

s[-3:] is ‘ace’ — beginning with the 3rd char from the last and extending to the last point of the string.


The slicing syntax is a convenient means to refer to sub-parts of sequences. This typically includes strings and lists. If you want to know more about strings, substrings, obtaining substrings by slicing Python strings, how to use the slice notation in Python, etc. make sure to contact Softlogic. We provide career-oriented training and also thorough placement assistance

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