String Methods

Section 11.5 String Methods

We previously used a few functions like print() and len() where we place the variable inside the parentheses such as len(my_str). On the other hand, a method is a function that is attached to a specific Python object. To access this function, we write the object, then a dot ., and then the name of the method. The "dot notation" is the way we connect the name of an object to the name of a method it can perform. For example, we can write my_str.upper() when we want the string my_str to perform the upper() method to create an upper-case version of itself.

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Remember that strings are immutable. Therefore, all string methods give us a new string and must be assigned to a new variable. The original string is unchanged.

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In this example, upper is a method that can be invoked on any string object to create a new string in which all the characters are in uppercase.

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In addition to upper, the following table provides some useful string methods.

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Table 11.5.1.

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Method	Parameters	Description
upper	none	Returns a string in all uppercase
lower	none	Returns a string in all lowercase
count	item	Returns the number of occurrences of item
replace	old, new	Replaces all occurrences of old substring with new
index	item	Returns the leftmost index where the substring item is found, or error if not found
strip	none	Returns a string with the leading and trailing whitespace (e.g.,space, tab, new line) removed
lstrip	none	Returns a string with the leading whitespace removed
rstrip	none	Returns a string with the trailing whitespace removed
split	optional string as separator	Breaks up the string into a list of individual words/strings

You should experiment with these methods so that you understand what they do. Note once again that the methods that return strings do not change the original. You can also consult the Python documentation for strings for a more comprehensive list of string methods and their descriptions.

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Check your understanding

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Checkpoint 11.5.2.

What is printed by the following statements?

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s = "python rocks"
print(s.count("o") + s.count("p"))

0
There are definitely o and p characters.
2
There are 2 o characters but what about p?
3
Yes, add the number of o characters and the number of p characters.

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Checkpoint 11.5.3.

What is printed by the following statements?

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s = "python rocks"
print(s[1] * s.index("n"))

yyyyy
Yes, s[1] is y and the index of n is 5, so 5 y characters. It is important to realize that the index method has precedence over the repetition operator.
55555
Close. 5 is not repeated, it is the number of times to repeat.
n
This expression uses the index of n
Error, you cannot combine all those things together.
This is fine, the repetition operator used the result of indexing and the index method.

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Two of the most useful methods on strings involve lists of strings. The split method breaks a string into a list of words. By default, any number of whitespace characters is considered a word boundary.

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An optional argument called a delimiter can be used to specify which characters to use as word boundaries.

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Notice that the delimiter doesn’t appear in the result. The inverse of the split method is join. You choose a desired separator string, (often called the glue) and join the list with the glue between each of the elements.

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The list that you glue together (wds in this example) is not modified. Also, you can use empty glue or multi-character strings as glue. Check your understanding

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Checkpoint 11.5.4.

Create a variable called words and assign to it a list whose elements are the words in the string str1.

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Checkpoint 11.5.5.

Create a variable called emotions and assign to it a list whose elements are the words in the string str1.

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You have attempted of activities on this page.

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