01_Text_and_Bytes — Python's text model

Level: 101 → 301 · for Python programmers

Python 3's defining decision is that text and bytes are different types and it will not convert between them behind your back. Everything in this chapter follows from that one choice: why len() gives two answers, why open() is a bet, why a Polish name list sorts wrong, and why a filename is not quite a string.

This is the chapter to read first — not because text is the most important part of Python, but because it is the part where a wrong mental model survives longest before failing, and it fails in production on data from someone else's country.

#	Lesson	The question it answers	Status
1	str is not bytes	Which type am I holding, and why won't Python mix them?	written
2	String literals	What do the r, b and f prefixes change — and why is '\d' two characters?	written
3	Encode and decode	Which direction is which, and what does errors= throw away?	written
4	Making a bytes object	Why is bytes(5) five zero bytes and bytes([5]) one?	written
5	bytearray is the mutable one	Which of the two binary types can I write into, and what does mutability cost?	written
6	Counting characters	How long is this string — and which of the four answers did you want?	written
7	Slicing is not indexing	Why does s[100] raise when s[:100] does not — and what does s[::-1] do to an accent?	written
8	Is it a letter?	What do the twelve is* predicates actually test?	written
9	Lowercasing is not folding	Why do 'straße' and 'STRASSE' still differ after .lower() — and what is casefold() for?	written
10	repr is not str	Why is a space "printable" and a tab not — and what did str(b'x') just do?	written
11	What ends a line	Which characters count as a line break — and why do I get three different answers?	written
12	strip is a set, not a prefix	Why did lstrip('Arthur: ') eat three more characters than I asked for?	written
13	Four ways to find it	Why did a search that found nothing hand back the last character of my string?	written
14	translate is a table, keyed by ordinal	Why is maketrans a separate call, and why is translate one pass where three .replace() calls are not?	written
15	The format mini-language	Is f'{x:>8}' the same grammar as '{:>8}'.format(x) — and where does % fit?	written
16	Padding is not alignment	Why is my column still ragged after ljust(20) — and why did zfill keep the minus sign?	written
17	Comparison has a mode	Which of the four ways to compare two strings did I just use?	written
18	Normalization	Why can 'é' == 'é' be False?	stub
19	Sorting is not comparing	Why does sorted() put Łukasiewicz after Zawadzki?	written
20	Opening a file	What does open() decide for me besides the encoding?	written
21	Standard in, standard out, and pipes	Why does piping my program into head raise BrokenPipeError — and how do I write bytes to stdout?	stub
22	Filenames are not text	How does Python read a filename that isn't valid UTF-8?	written
23	What kind of file is this?	Which of the five APIs answers the question I actually asked?	written
24	The string module	What is left in a module named string, and why is capwords not title?	written
25	The codecs registry	What is in the registry that .encode() and .decode() cannot reach — and why does decoding a stream in chunks differ?	written
26	bin() is not the bits	Why is bin(-9) '-0b1001' — and where did the bits go?	written

Several of these pages end with a ## Practice section — a kata: predict the answer, then run it, then check. They are indexed in KATAS.md, which is also the only place they are put in an order, and that order is not this one: it puts the four bytes katas first, because that is the thing this chapter's readers report bouncing off.

Where this sits relative to the other libraries

The encodings library ↗ teaches encodings as a subject, using four languages to illustrate. This chapter teaches Python's answer to that subject — the type boundary, the codec registry, the errors= policies, the locale coupling. Where a page here needs the language-agnostic groundwork, it links there rather than repeating it. The crosswalk is the index of which idea lives where.