Strings, Indexing and Slicing

Text is everywhere: names, messages, URLs, CSV rows, chatbot replies. In Python, text is a str – an ordered sequence of characters. Once you can pick characters out of a string and cut it into pieces, a huge range of everyday problems becomes easy.

Every character has a position

Positions (indexes) start at 0. Python also counts from the end with negative indexes: -1 is the last character.

word = "PYTHON"
print(word[0])
print(word[3])
print(word[-1])
print(word[-2])
print(len(word))

P
H
N
O
6

The last valid index is always len(word) - 1. Asking for word[6] raises IndexError: string index out of range.

Slicing: taking a piece

A slice text[start:stop] returns the characters from start up to, but not including, stop – the same rule as range(). Either side can be left out.

word = "PYTHON"
print(word[0:2])    # from 0 up to 2
print(word[2:])     # from 2 to the end
print(word[:4])     # from the start up to 4
print(word[-3:])    # last three
print(word[:])      # a full copy

PY
THON
PYTH
HON
PYTHON

Slices never raise IndexError – out-of-range values are simply clipped:

print("hi"[0:100])
print("hi"[5:])     # empty string

hi

The step

A third number sets the step. A negative step walks backwards – which gives the famous one-line reverse:

digits = "0123456789"
print(digits[::2])     # every second character
print(digits[1::3])    # start at 1, every third
print(digits[::-1])    # reversed

word = "level"
print(word == word[::-1])   # palindrome check

02468
147
9876543210
True

Strings are immutable

You cannot change a character inside a string. Any "change" builds a new string:

name = "kavya"
# name[0] = "K"   → TypeError: 'str' object does not support item assignment
name = "K" + name[1:]
print(name)
print(name.upper(), name)   # methods return new strings, too

Kavya
KAVYA Kavya

Joining, repeating and checking

first, last = "Ravi", "Kumar"
print(first + " " + last)
print("-" * 20)
print("Kum" in last)
print(len(first + last))

for ch in "abc":
    print(ch.upper(), end="")

Ravi Kumar
--------------------
True
9
ABC

Escape characters

A backslash gives the next character a special meaning:

print("Name:\tAsha\nCity:\tPune")
print('It\'s fine')
print("C:\\Users\\asha")

Name:	Asha
City:	Pune
It's fine
C:\Users\asha

Raw strings

Put r before the quotes and backslashes are kept as they are. Use raw strings for Windows paths and regular expressions:

print(r"C:\new\test")
print("C:\new\test")    # \n and \t are interpreted!

C:\new\test
C:
ew	est

Multi-line strings

message = """Dear Customer,
Your order has shipped.
Thank you!"""
print(message)
print(message.count("\n") + 1, "lines")

Dear Customer,
Your order has shipped.
Thank you!
3 lines

Characters and numbers

Every character has a Unicode number. ord() gives the number and chr() goes back – useful for simple ciphers:

print(ord("A"), ord("a"), chr(66))

secret = ""
for ch in "HAL":
    secret += chr(ord(ch) + 1)   # shift each letter by one
print(secret)

65 97 B
IBM

Practice

Summary