Python strings, slicing and f-strings

Strings, slices and f-strings

A string is a sequence of characters, and each character has a position called an index. Python counts from 0, not 1.

word = "python"
#       012345
word[0]    # 'p'
word[5]    # 'n'
word[-1]   # 'n'  negative indexes count from the end

len(word) gives the length, 6. Asking for word[6] is an IndexError: the last index is always len - 1.

A slice grabs a chunk: word[start:stop]. It includes start but stops before stop. So word[0:2] is "py". Leave a side empty to mean "from the beginning" or "to the end": word[2:] is "thon", word[:-1] is "pytho".

Strings come with methods, functions you call with a dot:

s.upper() and s.lower() change case.
s.strip() removes spaces from both ends.
s.replace("a", "b") swaps text.
s.count("o") counts how many times something appears.

Here is the catch: strings are immutable. Methods never change the original; they give you back a new string. name.upper() on its own line does nothing useful. You need name = name.upper() to keep the result.

To build text from variables, use an f-string: put f before the quote and drop variables into { }.

age = 15
print(f"I am {age} and next year I'll be {age + 1}")

You can put any expression inside the braces, and {price:.2f} shows a number with exactly 2 decimal places. Way nicer than gluing strings with +, which also refuses to mix strings and numbers.

Try it yourself

Edit it. Break it. Run it again.

word = "python"
print(word[0], word[-1])     # first and last letters
print(word[0:2], word[2:])   # slices
print(len(word))

shout = word.upper()         # word itself is unchanged
print(word, shout)

name = "  Sam  "
print("[" + name.strip() + "]")

price = 4.5
print(f"{word} costs {price:.2f} and has {len(word)} letters")

Your turn

Type it yourself. That is the whole trick.

1Initials

Using indexing on first and last, print the initials of the name, with a dot after each, so the output is exactly A.L. Use an f-string.

first = "Ada"
last = "Lovelace"
print(f"{first}.{last}.")

2Tidy username

raw is a messy username. Strip the spaces off both ends and make it lowercase, then print it, so the output is exactly gamer_kid. Remember that string methods return a new string.

raw = "   GaMeR_KiD  "
raw.strip()
print(raw)