Why doesn t calling a Python string method do anything unless you assign its output

Question

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 hello to goodbye  thus it should change the string  hello world  to  goodbye world   But X just remains  hello world   Why is my code not working

User · Answer

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

User · Answer

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     quot program c quot    quot stdio hpp quot    quot sample hpp quot    quot a out quot    quot math hpp quot    quot hpp out quot     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  quot  hpp quot            x   i replace  quot hpp quot    quot h quot           newfilenames append x      else          newfilenames append i    print newfilenames    Should be   quot program c quot    quot stdio h quot    quot sample h quot    quot a out quot    quot math h quot    quot hpp out quot

User · Answer

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

[python] Why doesn't calling a Python string method do anything unless you assign its output?

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