Learning python - Methods

What is a method?

A method is a function that belongs to a value. You call it with a dot after the value or variable name.

greeting = "hello"
capitalised = greeting.capitalize()
print(capitalised)
# Output: Hello

capitalize() is a method that belongs to the string stored in greeting. It returns a new version of the string with the first letter made uppercase.

Methods do not always change the original value. For strings, methods like upper() return a new string, because strings are immutable.

Using dir() to see available methods

You can use dir() on a value or variable to see which methods it has.

greeting = "hello"
print(dir(greeting))

This shows a list of methods you can use with strings.

Methods and brackets ()

Methods are written with brackets () at the end. Some methods need arguments inside the brackets.

text = "hello hello"
print(text.count("hello"))
# Count how many times "hello" appears

Here, "hello" is an argument to count().

Some methods need more than one argument:

text = "I like cats"
new_text = text.replace("cats", "dogs")
print(new_text)
# Output: I like dogs

replace() needs two arguments: what to replace, and what to replace it with.

String methods

Checking what a string contains

These methods return True or False.

code = "12345"
print(code.isdigit())   # True

name = "Alice"
print(name.isalpha())   # True

space_only = "   "
print(space_only.isspace())  # True

isdigit() checks if all characters are digits.
isalpha() checks if all characters are letters.
isspace() checks if the string only contains whitespace (spaces, tabs, new lines).

These are useful when validating user input.

Splitting strings

text = "apples,bananas,oranges"
parts = text.split(",")
print(parts)
# Output: ['apples', 'bananas', 'oranges']

split() breaks a string into a list, using the separator you give it.

lines = "First line\nSecond line\nThird line"
print(lines.splitlines())
# Output: ['First line', 'Second line', 'Third line']

splitlines() splits the string at line breaks.

Joining strings

words = ["Python", "is", "fun"]
sentence = " ".join(words)
print(sentence)
# Output: Python is fun

join() does the opposite of split(). It takes a list of strings and joins them into one string, using the string before .join() as the separator.

Removing spaces

text = "  hello  "
print(text.strip())
# 'hello'

print(text.lstrip())
# 'hello  '

print(text.rstrip())
# '  hello'

strip() removes spaces from both ends.
lstrip() removes spaces from the left (start).
rstrip() removes spaces from the right (end).

This is useful when cleaning up user input or data from files.

Finding text

message = "welcome to python"
print(message.find("to"))
# Output: 8

find() returns the index where the text starts, or -1 if it is not found.

print(message.index("to"))
# Output: 8

index() is similar, but it raises an error if the text is not found.

Use find() when you want a safe search, and index() when not finding the text should be treated as an error.

Replacing text

sentence = "I like blue cars and blue bikes"
fixed = sentence.replace("blue", "red")
print(fixed)
# Output: I like red cars and red bikes

replace() changes all matches of one substring to another.

Counting text

note = "test, test, test"
print(note.count("test"))
# Output: 3

count() tells you how many times a substring appears.

Checking start and end

filename = "report.pdf"
print(filename.startswith("report"))  # True
print(filename.endswith(".pdf"))      # True

startswith() checks if a string begins with the given text.
endswith() checks if a string finishes with the given text.

These are useful for checking file types, prefixes, or simple patterns.

Lambda functions

A lambda function is a tiny function you write in one line.
It does not need a name and is often used when you only need the function once.

double = lambda number: number * 2
print(double(5))
# Output: 10

This does the same job as:

def double(number):
    return number * 2

Lambdas are useful when you want a quick, simple function without creating a full def block.

map()

map() takes a function and applies it to every item in a list.

numbers = [1, 2, 3, 4]

def add_one(n):
    return n + 1

new_list = list(map(add_one, numbers))
print(new_list)
# Output: [2, 3, 4, 5]

You can use map() with a lambda instead:

new_list = list(map(lambda n: n + 1, numbers))

Many people prefer list comprehensions because they are easy to read:

new_list = [n + 1 for n in numbers]

Even if you don’t use lambda or map() often, it helps to recognise them in other people's code.

Vivienne Reay