Strings and Encoding

Character Encodings


  • Files (and networks, and …) contain arbitrary bytes

  • Files don’t have an idea of their content

  • ⟶ Content can be anything

    • Raw bytes

    • Plain 7-bit ASCII

    • ISO 8859-1

    • One of 2156 Chinese (multibyte) character sets

    • One of 1375 Japanese (multibyte) character sets

    • UTF-8, UTF-16, UTF-32

    • Many many more …


  • Unicode — one encoding to rule them all

  • Internally, Python strings are sequences of Unicode code points

Strings and Encodings

Where does the data come from and go to?

  • Programmer has to know what the source contains, and act accordingly

  • Raw bytes ⟶ create bytes objects

  • Strings ⟶ which encoding?

    • Email: MIME headers (⟶ email module)

    • Files: specify encoding parameter at file object creation (⟶ later)

    • Otherwise: read byte data and convert to string objects

At the programmer’s responsibility!

  • Has always been programmer’s responsibility

  • Python 3 just doesn’t let you mix str and bytes

From Raw Bytes to Strings (1)

Pre-Unicode: ISO/IEC 8859-1 (“Latin-1”) for Mid-European alphabet

Jörg, as read from a file with unknown encoding
>>> joerg_raw = b'J\xf6rg'
>>> type(joerg_raw)
<class 'bytes'>
  • File happens to be Latin-1 encoded

  • \xf6 is “ö” in Latin-1

  • … but that information isn’t there ⟶ binary

From Raw Bytes to Strings (2)

Transformation to string should be done as early as possible

  • Everything’s clear if one knows what’s in

  • ⟶ Transformation to Unicode (rules them all)

  • ⟶ Nobody has to know anymore what’s in

Transfer raw bytes into string
>>> joerg = str(joerg_raw, encoding='iso-8859-1')
>>> type(joerg)
<class 'str'>
>>> joerg

From Strings to Raw Bytes

Internal string representation is Unicode

  • No-one cares (has to care)

  • Unicode is a set of numbers, not a concrete encoding

“ö” is obviously multibyte in UTF-8
>>> joerg.encode('utf-8')
“ö” is unknown in China
>>> joerg.encode('big5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'big5' codec can't encode ....

Source File Encoding

Question: how are string literals encoded?

  • Default: ASCII

  • ⟶ umlauts not properly encoded in strings

  • Unless otherwise specified

Explicit source encoding
# -*- encoding: utf-8 -*-