Working with strings

Strings support a lot of built-in operations. Concatenation:

first = "Ada"
last = "Lovelace"
full = first + " " + last
print(full)  # Ada Lovelace

But the tool you'll reach for constantly is an f-string, which lets you embed variables directly inside text:

age = 28
print(f"{full} is {age} years old")
# Ada Lovelace is 28 years old

Useful string methods:

"Hello".lower()      # 'hello'
"Hello".upper()       # 'HELLO'
"  hi  ".strip()      # 'hi'
"a,b,c".split(",")    # ['a', 'b', 'c']
len("Hello")           # 5

Strings are indexed starting at 0, and you can slice them:

word = "Python"
word[0]      # 'P'
word[0:3]    # 'Pyt'
word[-1]     # 'n' (last character)
word[::-1]   # 'nohtyP' -- a step of -1 reverses the string

Strings are immutable

You can't change a character in place -- word[0] = "J" raises a TypeError. Every string method that looks like it "changes" a string (.upper(), .replace(), .strip()) actually returns a brand-new string and leaves the original untouched:

text = "hello"
text.upper()
print(text)  # still "hello" -- upper() didn't change it in place

text = text.upper()  # you have to reassign to keep the result
print(text)  # "HELLO"

This trips up a lot of beginners: calling a string method and expecting the original variable to have changed, then being confused when it hasn't.

More string methods worth knowing

"hello world".title()          # 'Hello World'
"hello world".replace("world", "there")  # 'hello there'
"hello".startswith("he")        # True
"hello".endswith("lo")           # True
"hello".find("l")                 # 2 -- index of first match, -1 if not found
",".join(["a", "b", "c"])          # 'a,b,c'

join is the reverse of split -- it's a method on the separator string, which surprises people the first time they see it, since you might expect it to be a method on the list instead.

Formatting numbers inside f-strings

price = 19.999
print(f"${price:.2f}")   # $20.00 -- round to 2 decimal places
count = 7
print(f"{count:03}")      # 007 -- pad with zeros to width 3

The part after the colon inside {} is a format spec -- .2f means "fixed-point, 2 decimal places," and it's the standard way to control how numbers display without manually rounding or padding them yourself.

Comparing f-strings to .format() and %

You may see two older styles in existing code: "{} is {}".format(name, age) and "%s is %s" % (name, age). Both still work, but f-strings are the current standard -- they're shorter and let you see the variable name directly inside the string instead of matching up positions by hand.