Strings: working with text

Python counts the letters of "Python" from 0, so "Python"[5] is the last letter, n, and "Python"[6] is an error.

A name tag needs text built from pieces

Think of the name tags at a school event or a club meeting. Every tag has the same layout and a different name, so a program can print them all from one pattern. Text in Python is a string, and the quickest way to build one out of pieces is an f-string: put the letter f in front of the opening quote and write variable names inside curly braces. Python swaps each {name} for the value of that variable.

name = "Mina"
club = "Robotics"
print(f"Hello, {name}!")
print(f"{name} joined {club}.")

Hello, Mina!
Mina joined Robotics.

There are two older ways to join text. The + operator glues strings end to end, and * repeats a string. Both sides of + must be strings, so a number has to be turned into text with str() first. f-strings do that conversion for you, which is one reason they are the usual choice.

item = "gimbap"
count = 2
print(item + " x " + str(count))
print("-" * 12)
print(f"{item} x {count}")

gimbap x 2
------------
gimbap x 2

Each character has a position number, starting at 0

A string is a row of characters, and each one has an index. The first character is at index 0, not 1. Square brackets pick one character: word[0] is the first. Negative indexes count from the end, so word[-1] is always the last character, whatever the length. len() gives the number of characters, spaces and punctuation included.

word = "Python"
print(word[0], word[5], word[-1])
print(len(word))
print(word[0:2], word[2:])

P n n
6
Py thon

word[0:2] is a slice: it starts at index 0 and stops just before index 2, giving "Py". Leave out the end, as in word[2:], and the slice runs to the end of the string. The stop-before rule looks odd at first, but it means word[0:2] + word[2:] always gives back the whole word.

String methods return a new string

Strings come with built-in tools called methods, written after a dot. upper() and lower() change the case, replace(old, new) swaps every copy of one piece of text for another, and strip() removes spaces from both ends, which helps when someone types an extra space into a form. The keyword in checks whether one string appears inside another and gives True or False. None of these change the original string. A string cannot be changed after it is made; each method hands back a new one, and you keep it by assigning it to a name.

menu = "  tteokbokki  "
clean = menu.strip()
print(clean.upper())
print(clean.replace("k", "K"))
print("bok" in clean, len(menu), len(clean))

TTEOKBOKKI
tteoKboKKi
True 14 10

To put a quote mark inside a string, use the other kind of quote around it: "It's fine". A backslash starts an escape sequence: \n inside a string means "start a new line here", so one print can show two lines.

print("It's lunch time.")
print("Line one\nLine two")

It's lunch time.
Line one
Line two