Solving problem is about exposing yourself to as many situations as possible like Why doesn’t calling a Python string method do anything unless you assign its output? and practice these strategies over and over. With time, it becomes second nature and a natural way you approach any problems in general. Big or small, always start with a plan, use other strategies mentioned here till you are confident and ready to code the solution.
In this post, my aim is to share an overview the topic about Why doesn’t calling a Python string method do anything unless you assign its output?, which can be followed any time. Take easy to follow this discuss.
I try to do a simple string replacement, but I don’t know why it doesn’t seem to work:
X = "hello world" X.replace("hello", "goodbye")
I want to change the word
goodbye, thus it should change the string
"hello world" to
"goodbye world". But X just remains
"hello world". Why is my code not working?
This is because strings are immutable in Python.
Which means that
X.replace("hello","goodbye") returns a copy of
X with replacements made. Because of that you need replace this line:
with this line:
X = X.replace("hello", "goodbye")
More broadly, this is true for all Python string methods that change a string’s content “in-place”, e.g.
You must assign their output to something if you want to use it and not throw it away, e.g.
X = X.strip(' t') X2 = X.translate(...) Y = X.lower() Z = X.upper() A = X.join(':') B = X.capitalize() C = X.casefold()
and so on.
All string functions as
strip are returning a string without modifying the original. If you try to modify a string, as you might think
well it is an iterable, it will fail.
x = 'hello' x = 'i' #'str' object does not support item assignment
There is a good reading about the importance of strings being immutable: Why are Python strings immutable? Best practices for using them
Example for String Methods
Given a list of filenames, we want to rename all the files with extension hpp to the extension h. To do this, we would like to generate a new list called newfilenames, consisting of the new filenames. Fill in the blanks in the code using any of the methods you’ve learned thus far, like a for loop or a list comprehension.
filenames = ["program.c", "stdio.hpp", "sample.hpp", "a.out", "math.hpp", "hpp.out"] # Generate newfilenames as a list containing the new filenames # using as many lines of code as your chosen method requires. newfilenames =  for i in filenames: if i.endswith(".hpp"): x = i.replace("hpp", "h") newfilenames.append(x) else: newfilenames.append(i) print(newfilenames) # Should be ["program.c", "stdio.h", "sample.h", "a.out", "math.h", "hpp.out"]