There is a collection of methods that are designed to operate on strings. All of these methods are applied to a string to perform some operation. Since strings are immutable, they never change the string they work on, but they always return a changed version of the string.

Like the format() method introduced in the chapter “Simple functions”, all these methods are called using the syntax <string>.<method>(), i.e., you have to write the string that they work on before the method call, with a period in between. You will encounter this more often, and why this is implemented in this way will be explained later in the course, in the chapter about object orientation.

Most of these methods are not part of a specific module, but can be called without importing them. There is a string module that contains specific constants and methods that can be used in your programs, but the methods I discuss here can all be used without importing the string module.

strip()

strip() removes from a string leading and trailing spaces, including leading and trailing newlines and other characters that may be viewed as spaces. If instead of spaces, you want to remove different characters, you can add a string parameter that consists of all characters you want to be removed.

s = "    And now for something completely different \n    "
print( "["+s+"]" )
s = s.strip()
print( "["+s+"]" )

upper() and lower()

upper() creates a version of a string of which all letters are capitals. lower() is equivalent, but uses only lower case letters. Neither method uses parameters.

s = "The Meaning of Life"
print( s )
print( s.upper() )
print( s.lower() )

find()

find() can be used to search in a string for the starting index of a particular substring. As parameters it gets the substring, and optionally a starting index to search from, and an ending index. It returns the lowest index where the substring starts, or -1 if the substring is not found.

s = "Humpty Dumpty sat on the wall"
print( s.find( "sat" ) )
print( s.find( "t" ) )
print( s.find( "t", 12 ) )
print( s.find( "q" ) )

replace()

replace() replaces all occurrences of a substring with another substring. As parameters it gets the substring to look for, and the substring to replace it with. Optionally, it gets a parameter that indicates the maximum number of replacements to be made.

I must stress again that strings are immutable, so the replace() function is not actually changing the string. It returns a new string that is a copy of the string with the replacements made.

s = 'Humpty Dumpty sat on the wall'
print( s.replace( 'sat on', 'fell off' ) )

split()

split() splits a string up in words, based on a given character or substring which is used as separator. The separator is given as the parameter, and if no separator is given, the white space is used, i.e., you split a string in the actual words (though punctuation attached to words is considered part of the words). If there are multiple occurrences of the separator next to each other, the extra ones are ignored (i.e., with the white space as separator, it does not matter if there is a single white space between two words, or multiple).

The result of this split is a so-called “list” of words. Lists are discussed in a coming chapter, so for now I will not say much about them. I just indicate that if you want to access the separate words, you can use the for <word> in <list>: construction.

s = 'Humpty Dumpty      sat    on the wall   '
wordlist = s.split()
for word in wordlist:
    print( word )

A very useful property of splitting is that we can decode some basic file formats. For example, a comma separated value (CSV) file is a very simple format, of which the basic setup is that each line consists of values that are separated by a comma. These values can be split from each other using the split() method.¹¹

csv = "2016,September,28,Data Processing,Tilburg University"
values = csv.split( ',' )
for value in values:
    print( value )

join()

join() is the opposite of split(). join() joins a list of words together, separated by a specific separator. This sounds like it would be a method of lists, but for historic reasons it is defined as a method for strings. Since all string methods are called using the format <string>.<method>(), there must be a string in front of the call to join(). That string is the separator that you want to use, while the parameter of the method is the list that you want to join together. The return value, as always, is the resulting string.

s = "Humpty;Dumpty;sat;on;the;wall"
wordlist = s.split( ';' )
s = " ".join( wordlist )
print( s )

Practice

In the string "How much woot would a wootchuck chuck if a wootchuck could chuck woot." the word "wood" is misspelled. Use replace() to replace all occurrences of this spelling error with the correct spelling.

Display the contents of the string "Nobody expects the Spanish Inquisition!# In fact, those who do expect the Spanish Inquisition..." up to, but not including, the hash mark (#). Use find() to get the index of the hash mark.

Write a program that prints a “cleaned” version of all the words in a string. Everything that is not a letter should be removed and be considered a separator. All the letters should be lower case. For example, the string "I'm sorry, sir." should produce four words, namely "i", "m", "sorry", and "sir". You can use the function for string cleaning which you wrote as an exercise above.