Python strings

What is String in Python?

A string is a sequence of characters.

A character is simply a symbol. For example, the English language has 26 characters.

Computers do not deal with characters, they deal with numbers (binary). Even though you may see characters on your screen, internally it is stored and manipulated as a combination of 0s and 1s.

This conversion of character to a number is called encoding, and the reverse process is decoding. ASCII and Unicode are some of the popular encodings used.

In Python, a string is a sequence of Unicode characters. Unicode was introduced to include every character in all languages and bring uniformity in encoding. You can learn about Unicode from Python Unicode.

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How to create a string in Python?

Strings can be created by enclosing characters inside a single quote or double-quotes. Even triple quotes can be used in Python but generally used to represent multiline strings and docstrings.

# defining strings in Python

# all of the following are equivalent

# triple quotes string can extend multiple lines

When you run the program, the output will be:

Hello

Hello

Hello

Hello, welcome to

           the world of Python

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How to access characters in a string?

We can access individual characters using indexing and a range of characters using slicing. Index starts from 0. Trying to access a character out of index range will raise an IndexError. The index must be an integer. We can’t use floats or other types, this will result into TypeError.

Python allows negative indexing for its sequences.

The index of -1 refers to the last item, -2 to the second last item and so on. We can access a range of items in a string by using the slicing operator :(colon).

When we run the above program, we get the following output:

str =  programiz

str[0] =  p

str[-1] =  z

str[1:5] =  rogr

str[5:-2] =  am

If we try to access an index out of the range or use numbers other than an integer, we will get errors.

# index must be in range

IndexError: string index out of range

# index must be an integer

TypeError: string indices must be integers

Slicing can be best visualized by considering the index to be between the elements as shown below.

If we want to access a range, we need the index that will slice the portion from the string.

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How to change or delete a string?

Strings are immutable. This means that elements of a string cannot be changed once they have been assigned. We can simply reassign different strings to the same name.

TypeError: ‘str’ object does not support item assignment

We cannot delete or remove characters from a string. But deleting the string entirely is possible using the del keyword.

TypeError: ‘str’ object doesn’t support item deletion

NameError: name ‘my_string’ is not defined

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Python String Operations

There are many operations that can be performed with strings which makes it one of the most used data types in Python.

Concatenation of Two or More Strings

Joining of two or more strings into a single one is called concatenation.

The + operator does this in Python. Simply writing two string literals together also concatenates them.

The * operator can be used to repeat the string for a given number of times.

# Python String Operations

When we run the above program, we get the following output:

str1 + str2 =  HelloWorld!

str1 * 3 = HelloHelloHello

Writing two string literals together also concatenates them like + operator.

If we want to concatenate strings in different lines, we can use parentheses.

Iterating Through a string

We can iterate through a string using a for loop. Here is an example to count the number of ‘l’s in a string.

# Iterating through a string

When we run the above program, we get the following output:

3 letters found

String Membership Test

We can test if a substring exists within a string or not, using the keyword in.

Built-in functions to Work with Python

Various built-in functions that work with sequence work with strings as well.

Some of the commonly used ones are enumerate() and len(). The enumerate() function returns an enumerate object. It contains the index and value of all the items in the string as pairs. This can be useful for iteration.

Similarly, len() returns the length (number of characters) of the string.

str = ‘cold’

# enumerate()

When we run the above program, we get the following output:

list(enumerate(str) =  [(0, ‘c’), (1, ‘o’), (2, ‘l’), (3, ‘d’)]

len(str) =  4

Python String Formatting

Escape Sequence

If we want to print a text like He said, “What’s there?”, we can neither use single quotes nor double quotes. This will result in a SyntaxError as the text itself contains both single and double quotes.

One way to get around this problem is to use triple quotes. Alternatively, we can use escape sequences.

An escape sequence starts with a backslash and is interpreted differently. If we use a single quote to represent a string, all the single quotes inside the string must be escaped. Similar is the case with double quotes. Here is how it can be done to represent the above text.

When we run the above program, we get the following output:

He said, “What’s there?”

He said, “What’s there?”

He said, “What’s there?”

Here is a list of all the escape sequences supported by Python.

Escape Sequence	Description
\newline	Backslash and newline ignored
\\	Backslash
\’	Single quote
\”	Double quote
\a	ASCII Bell
\b	ASCII Backspace
\f	ASCII Formfeed
\n	ASCII Linefeed
\r	ASCII Carriage Return
\t	ASCII Horizontal Tab
\v	ASCII Vertical Tab
\ooo	Character with octal value ooo
\xHH	Character with hexadecimal value HH

Here are some examples

This is HEX representation

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Raw String to ignore escape sequence

Sometimes we may wish to ignore the escape sequences inside a string. To do this we can place r or R in front of the string. This will imply that it is a raw string and any escape sequence inside it will be ignored.

This is \x61 \ngood example

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The format() Method for Formatting Strings

The format() method that is available with the string object is very versatile and powerful in formatting strings. Format strings contain curly braces {} as placeholders or replacement fields which get replaced.

We can use positional arguments or keyword arguments to specify the order.

When we run the above program, we get the following output:

— Default Order —

John, Bill and Sean

— Positional Order —

Bill, John and Sean

— Keyword Order —

Sean, Bill and John

The format() method can have optional format specifications. They are separated from the field name using colon. For example, we can left-justify <, right-justify > or center ^ a string in the given space.

We can also format integers as binary, hexadecimal, etc. and floats can be rounded or displayed in the exponent format. There are tons of formatting you can use. Visit here for all the string formatting available with the format() method.

‘Binary representation of 12 is 1100’

‘Exponent representation: 1.566345e+03’

‘One third is: 0.333’

‘|butter    |  bread   |       ham|’

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Old style formatting

We can even format strings like the old sprintf() style used in C programming language. We use the % operator to accomplish this.

The value of x is 12.35

The value of x is 12.3457

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Common Python String Methods

There are numerous methods available with the string object. The format() method that we mentioned above is one of them. Some of the commonly used methods are lower(), upper(), join(), split(), find(), replace() etc. Here is a complete list of all the built-in methods to work with strings in Python.