Why doesn't calling a string method (such as .replace or .strip) modify (mutate) the string?
Question:
I tried this code to do simple string replacement:
X = "hello world"
X.replace("hello", "goodbye")
Why doesn’t X change, from "hello world" to "goodbye world"?
Answers:
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:
X.replace("hello", "goodbye")
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. replace,strip,translate,lower/upper,join,…
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 lower, upper, 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[0] = '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.
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"]
I tried this code to do simple string replacement:
X = "hello world"
X.replace("hello", "goodbye")
Why doesn’t X change, from "hello world" to "goodbye world"?
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:
X.replace("hello", "goodbye")
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. replace,strip,translate,lower/upper,join,…
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 lower, upper, 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[0] = '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.
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"]