String Methods

Methods

• Methods are functions that operate on a known piece of data
• Most operators are shortcuts to object methods

Example

>>> 1 + 2
3
>>> (1).__add__(2)
3

dir

• dir can be used to list methods available on an object

Example

>>> dir(1)
['__abs__', '__add__', ...]
>>> dir("")
['__add__', ...]

upper and lower

• upper can be used to uppercase a string
• lower can be used to lowercase it

Example

>>> a = "Hello, world!"
>>> a.lower()
'hello, world!'
>>> a.upper()
'HELLO, WORLD!'

How could we compare strings ignoring case?

Solution

color = input("Enter a color:")

if color.lower() == "red":
    print("Roses are red")
elif color.lower() == "blue":
    print("Violets are blue")
else:
    print("Unknown color")

find

• find can be used to get the position of a substring
• Returns the index of the start of the substring
• Returns -1 if substring is not found

Example

>>> a = "Hello, world!"
>>> a.find("world")
7

strip

• strip returns a copy of the string with leading and trailing whitespace removed

Example

>>> text = "    Hello, world!          "
>>> text.strip()
'Hello, world'

startswith

• startswith will return True if the string begins with the supplied prefix

Example

>>> text = "Hello, world!"
>>> text.startswith("Hello")
True
>>> text.startswith("world")
False

count

• count will count the number of non-overlapping substrings in some text

Example

>>> sentence = "The quick brown fox jumped over the lazy dog"
>>> sentence.count("o")
4

Slicing

• String indexing in Python supports returning substrings as well as individual characters
• A colon (:) is used to separate the start and end index

Example

>>> alphabet = "abcdefghijklmnopqrstuvwxyz"
>>> alphabet[2:4]
'cd'
>>> alphabet[0:5]
'abcde'
>>> alphabet[2:2]
''
>>> alphabet[2:3]
'c'

Negative Indexes

• In Python, it is legal for an index to be negative
• A negative index will index from the end of the object

Example

>>> text = "Hello, world!"
>>> text[-1]
'!'
>>> text[-2]
'd'

Inferred indices

• If a start index is left out, it is assumed to be the start
• If an end index is left out, it is assumed to be the end

Example

>>> text = "Hello, world"
>>> text[2:]
'llo, world!'
>>> text[:-2]
'Hello, worl'

Formatted String Literals

format

• Prior editions of the textbook used the format method on strings
• It is antiquated and has largely been superseded by f-strings since Python 3.6.
• You will not be required to use or understand the format method in this class

f-strings

• Provide simple string formatting
• Python expressions can be included within strings
• An f should be used before the opening quote of the string literal

Example

>>> f"The answer is {7*6}"

Example

name = input("What is you name?")

print(f"Hello, {name}")

Format Specifiers

• Can be added after an expression to adjust formatting
• This can be used for rounding or other purposes such as alignment

Example

num = 1

for pokemon in ["Charmander", "Charmeleon", "Charizard"]:
    print(f"{pokemon:12} {num}")
    num += 1

Example

import random

for _ in range(10):
    print(f"{random.random():.2f}")

Create a program that will capitalize the first word in a provided sentence.

Solution

sentence = "hello world"

sentence = sentence[0].upper() + sentence[1:]

print(sentence)

Create a program that will search a string for a word and display the word along with the 5 characters before and 5 characters after it.

Solution

sentence = "the quick brown fox jumped over the lazy dog"
word = "fox"

index = sentence.find(word)

print(sentence[index - 5: index + len(word) + 5])