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Day 3 of 14 30 minIndexingSlicingf-strings

Text is everywhere. Learn to index, slice, transform and format strings, the most-used type in Python.

By the end of today you can

  • Create strings with single, double and triple quotes
  • Read characters with positive and negative indexes
  • Cut out pieces of text with slicing
  • Use common string methods like strip, split and replace
  • Build readable output with f-strings

A string (str) is a sequence of characters: a name, a sentence, an email, a whole file. Today you'll learn to take strings apart and put them back together.

Making strings

Use single or double quotes; they mean the same thing. Triple quotes let a string span several lines.

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Mind map · click a dot to fold, drag to pan

Indexing: one character at a time

Every character has a position, called its index. Counting starts at 0. You can also count from the end with negative indexes.

Sketch
The word PYTHON in six boxes. Positive indexes 0 to 5 run left to right above the letters; negative indexes -6 to -1 run below. A bracket marks the slice s[1:4], covering Y, T and H.
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Slicing: cut out a piece

A slice s[start:stop] takes characters from start up to, but not including, stop. Leave a side empty to go to the beginning or end.

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SliceMeaning
s[a:b]From index a up to, not including, b
s[:b]The first b characters
s[a:]From a to the end
s[::step]Every step-th character; a step of -1 reverses

Strings can't be changed (immutable)

You can't change a character inside a string. String methods always return a new string, so assign the result if you want to keep it.

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Useful string methods

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MethodWhat it does
upper(), lower(), title()Change letter case
strip()Remove whitespace from both ends
replace(old, new)Replace every occurrence
split(sep)Break into a list of strings
sep.join(list)Join a list of strings with sep between
startswith(x), endswith(x)True or False
find(x)Index of the first match, or -1

f-strings: build text from values

Put an f before the quotes and write expressions inside curly braces. Add a format spec after a colon to control how values look.

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Recap

  • Strings are sequences of characters, written with '…', "…" or """…""".
  • Indexes start at 0; negative indexes count from the end.
  • s[start:stop] slices from start up to, not including, stop.
  • Strings are immutable: methods return new strings.
  • f-strings like f"{price:.2f}" are the clearest way to format output.

Knowledge check

Test what you learned

5 questions · pick an answer to see if you're right

0/5
  1. 1

    What does "python"[-1] return?

  2. 2

    What does "python"[1:4] return?

  3. 3

    You run name = "ada" and then name.upper(). What is name now?

  4. 4

    What does "a,b,,c".split(",") return?

  5. 5

    price = 4.5. Which f-string displays "Total: $4.50"?

Finished today's lesson?

Day 2Variables & Data TypesDay 4 Operators & Expressions