String class i.e str provides many useful methods to manipulate the string. Specifically, we will discuss methods which do the following.
- Search for a substring inside a string.
- Test strings.
- Format strings.
- Convert strings.
Recall from the earlier chapter that methods are functions which belongs to an object. However, unlike a function, a method is always called on an object using the following notation.
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1 |
object.method_name(arg1, arg2, arg3, ...., argN) |
Okay, Let’s get started.
Testing Strings
The following methods of the str class tests various types of characters inside the string.
| Method | Description |
|---|---|
str.isalnum() |
returns True if all the characters in the string is alphanumeric (a string which contains either number or alphabets or both). Otherwise False. |
str.isalpha() |
returns True if all the characters in the string are alphabets. Otherwise False. |
str.isdigit() |
returns True if all the characters in the string are digits. Otherwise False. |
str.islower() |
returns True if all the characters in the string are in lowercase. Otherwise False. |
str.isupper() |
returns True if all the characters in the string are in uppercase. Otherwise False. |
str.isspace() |
returns True if all the characters in the string are whitespace characters. Otherwise False. |
Here are some examples:
isalnum() method
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>>> >>> s = "A bite of python" >>> >>> s.isalnum() False >>> >>> "123".isalnum() True >>> >>> "abc".isalnum() True >>> >>> "abc123".isalnum() True >>> |
isalpha() method
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>>> >>> "123".isalpha() False >>> >>> "zyz".isalpha() True >>> >>> "$$$$".isalpha() False >>> >>> "abc233".isalpha() False >>> |
isdigit() method
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>>> >>> >>> "name101".isdigit() False >>> >>> "101".isdigit() True >>> >>> "101 ".isdigit() False >>> >>> "101.129".isdigit() False >>> |
islower() and isupper() method
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>>> >>> s 'A bite of python' >>> >>> s.islower() False >>> >>> "abc".islower() True >>> >>> s.isupper() False >>> >>> >>> "ABC".isupper() True >>> |
isspace() method
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>>> >>> "\n\t".isspace() True >>> >>> " \n\t".isspace() True >>> >>> "@ \n\t".isspace() False >>> >>> "123".isspace() False >>> |
Searching and Replacing Strings
The str class has following methods which allow you to search for a substring inside a string.
| Method | Description |
|---|---|
endswith(sub) |
Returns True if a string ends with substring sub. Otherwise False. |
startswith(sub) |
Returns True if string starts with substring sub. Otherwise False. |
find(sub) |
Returns the lowest index of the string where substring sub is found. If substring sub is not found -1 is returned. |
rfind(sub) |
Returns the highest index of the string where substring sub is found. If substring sub is not found -1 is returned. |
count(sub) |
It returns the number of occurrences of substring sub found in the string. If no occurrences found 0 is returned. |
replace(old, new) |
It returns a new string after replacing old substring with new. Notice that it does not change the object on which it is called. |
Some examples:
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>>> >>> s = "abc" >>> >>> s.endswith("bc") True >>> >>> "python".startswith("py") True >>> >>> "Learning Python".find("n") 4 >>> >>> "Learning Python".rfind("n") 14 >>> >>> "Learning Python".find("at") -1 >>> >>> "procrastination is the thief of time".count("ti") 3 >>> |
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>>> >>> >>> s1 = "Learning C" # old string >>> >>> id(s1) 49447664 # id of s1 >>> >>> s2 = s1.replace("C", "Python") # replace() creates a new string and assigns it to s2 >>> >>> s2 'Learning Python' >>> >>> id(s1) 49447664 # notice that s1 object is not changed at all >>> >>> |
Converting Strings
The following methods are commonly used to return a modified version of the string.
| Method | Description |
|---|---|
lower() |
Returns a new copy of the string after converting all of it's characters to lowercase. |
upper() |
Returns a new copy of the string after converting all of it's characters to uppercase. |
capitalize() |
Returns a new copy of the string after capitalizing only the first letter in the string. |
title() |
Returns a new copy of the string after capitalizing the first letter in each word. |
swapcase() |
Returns a new copy after converting lowercase letters to uppercase and vice-versa. |
strip() |
Returns a new copy of the string after removing all the leading and trailing whitespace characters. |
strip(chars) |
Returns a new copy of the string after removing chars from the beginning and end of the string. |
Always remember that these methods return a new string object and do not modify the original string object in any way.
Here are some examples:
lower() method
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>>> >>> "abcDEF".lower() 'abcdef' >>> >>> "abc".lower() 'abc' >>> |
upper() method
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>>> >>> "ABCdef".upper() 'ABCDEF' >>> >>> "ABC".upper() 'ABC' >>> |
capitalize() and title() method
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>>> >>> "a long string".capitalize() 'A long string' >>> >>> >>> "a long string".title() 'A Long String' >>> >>> |
swapcase() method
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>>> >>> "ABCdef".swapcase() 'abcDEF' >>> >>> "def".swapcase() 'DEF' >>> |
strip() method
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>>> >>> s1 = "\n\tName\tAge" >>> print(s1) Name Age >>> >>> >>> s2 = s1.strip() >>> >>> s2 'Name\tAge' >>> >>> print(s2) Name Age >>> >>> >>> s = "--Name\tAge--" >>> >>> s.strip("-") # return a new copy of string after removing - characters from beginning and end of the string 'Name\tAge' >>> >>> |
Formatting Methods
The following table list some formatting methods of the str class.
| Method | Description |
|---|---|
center(width) |
Returns a new copy of the string after centering it in a field of length width. |
ljust(width) |
Returns a new copy of the string justified to left in the field of length width. |
rjust(width) |
Returns a new copy of the string justified to right in the field of length width. |
center() method
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>>> >>> "NAME".center(20) ' NAME ' >>> >>> |
ljust() method
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>>> >>> "NAME".ljust(10) 'NAME ' >>> >>> "NAME".ljust(4) 'NAME' >>> >>> "NAME".ljust(5) 'NAME ' >>> |
rjust() method
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>>> >>> "NAME".rjust(10) ' NAME' >>> >>> "NAME".rjust(4) 'NAME' >>> >>> "NAME".rjust(5) ' NAME' >>> >>> |
