Strings in Python

Updated on May 20, 2019
Sam Shepards profile image

I'm a software developer with a great interest in data analysis and statistics.

Unlike other programming languages like C, C++ or Java, Python does not have the concept of a character. In fact, if we enclose a single character within single quotes, Python interpreter interprets it as a string with only one character. As discussed earlier, Python string values can be defined using either single or double quotes.

Python Strings are immutable. But the variable that points to a string in memory can be modified in such a way that it points to a different one. It is possible to use slice operator to make it seem like updating parts of the same string, but in reality, all that python interpreter does is to make the string variable point to a new string in memory.

#!/usr/bin/python3

firstString = "Hello"
secondString = 'World'
print("firstString: ", firstString)
print("secondString: ", secondString)
secondString = secondString[0:1] + 'alter'
print("secondString after modification: ", secondString)

The output of the above program will be:

firstString:  Hello
secondString:  World
secondString after modification:  Walter

Note how the result of concatenation is assigned explicitly to secondString at line number 7. If we don’t assign the value to secondString, the value printed for the last print statement will be “World”. This is because strings in python are immutable and doing a concatenation using + operation actually creates a new string in memory without altering the existing string objects.

Operators in String

In the previous section, we saw one special operator + which was used to concatenate two string values. We also saw how to use slice operators on String. We will learn about the remaining important special operators on Python string in this section.

The Repetition Operator(*)

It creates a new string after repeating the original string on the LHS of the asterisk as many times as denoted by the number following it.

The In And Not In Operators

These membership operators can be used to find out whether a given character is part of the string or not.

The % Operator

This performs string formatting.

The Raw String Operator(r/R)

Usually, when we use \ before certain characters, they may be translated to escape characters. In order to print these escape character strings we can use R or r just before the sequence.

#!/usr/bin/python3
firstString = "Hello"
secondString = 'World'
# Repetition operator
print(firstString * 2)

# Membership operator
if 'e' in firstString:
    print("Found letter e in firstString value")
else:
    print("Did not find letter e in firstString value")
if 'a' not in secondString:
    print("secondString value doesnt contain letter a")
else:
    print("secondString value contains letter a")
    
# prints hexadecimal value of 1240 using % formatting operator
print("1240 in hex is %x" % 1240)

print("Newline is represented by: ", r'\n')

Output for the above code will be:

HelloHello
Found letter e in firstString value
secondString value doesnt contain letter a
1240 in hex is 4d8
Newline is represented by:  \n

Important String Functions

There are many built-in functions in Python that helps one to manipulate strings. Some of the important ones are:

  • isalpha(), isalnum() and isdigit() : returns true if all characters in strings are alphabetic, alphanumeric and numeric respectively.
  • Islower() and isupper() – returns true if all characters are lowercase and uppercase respectively.
  • join(sequence) – returns a string formed by concatenating the strings in sequence passed. The different strings will be separated by the string on which the join operation is performed.
  • lower(), capitalize() and upper() – converts the given string to lower case form, capitalized form ( only first letter remains capitalized) and upper cases respectively.
  • Decode() and encode() – decodes and encodes given string using the encoding format.
  • len() – returns the length of the string.
  • lstrip() – trims the given string by removing leading whitespaces.
  • Max() and min() – returns maximum and minimum characters in the given string.

startswith() and endswith() – checks if a given string starts or ends with the given string sequence.

#!/usr/bin/python3
firstString = "hello"
print(firstString.upper())
print("firstString.islower()? ",firstString.islower())

separator = '_'
seq = ["This", "is", "a", "python", "tutorial"]
newStr = separator.join(seq)
print(newStr)
print("length of new string : ", len(newStr))

str2 = "      STRING WITH LEADING SPACE"
print(str2.lstrip())

if newStr.startswith("This"):
    print("newStr formed using join operation starts with This")
else:
    print("newStr formed using join operation does not start with This")

Output for the above program will be:

HELLO
firstString.islower()?  True
This_is_a_python_tutorial
length of new string :  25
STRING WITH LEADING SPACE
newStr formed using join operation starts with This

© 2019 Sam Shepards

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