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parse() is the opposite of format()

Project description

Parse strings using a specification based on the Python format() syntax.

parse() is the opposite of format()

Basic usage:

>>> from parse import *            # only exports parse() and compile()
>>> parse("It's {}, I love it!", "It's spam, I love it!")
<Result ('spam',) {}>
>>> p = compile("It's {}, I love it!")
>>> print p
<Parser "It's {}, I love it!">
>>> p.parse("It's spam, I love it!")
<Result ('spam',) {}>

Format Syntax

A basic version of the Format String Syntax is supported with anonymous (fixed-position), named and formatted fields:

{[field name]:[format spec]}

Field names must be a single Python identifier word. No attributes or element indexes are supported (as they would make no sense.)

Numbered fields are also not supported: the result of parsing will include the parsed fields in the order they are parsed.

The conversion of fields to types other than strings is done based on the type in the format specification, which mirrors the format() behaviour. There are no “!” field conversions like format() has.

Some simple parse() format string examples:

>>> parse("Bring me a {}", "Bring me a shrubbery")
<Result ('shrubbery',) {}>
>>> r = parse("The {} who say {}", "The knights who say Ni!")
>>> print r
<Result ('knights', 'Ni!') {}>
>>> print r.fixed
('knights', 'Ni!')
>>> r = parse("Bring out the holy {item}", "Bring out the holy hand grenade")
>>> print r
<Result () {'item': 'hand grenade'}>
>>> print r.named
{'item': 'hand grenade'}

Format Specification

Most of the Format Specification Mini-Language is supported:


The align operators will cause spaces (or specified fill character) to be stripped from the value. The alignment character “=” is not yet supported.

The comma “,” separator is not yet supported.

The types supported are a slightly different mix to the format() types. Some format() types come directly over: d, n, f, b, o, h, x and X. In addition some regular expression character group types D, w, W, s and S are also available.

The format() types %, F, e, E, g and G are not yet supported.

Type Characters Matched Output
w Letters and underscore str
W Non-letter and underscore str
s Whitespace str
S Non-whitespace str
d Digits (effectively integer numbers) int
D Non-digit str
n Numbers with thousands separators (, or .) int
f Fixed-point numbers float
b Binary numbers int
o Octal numbers int
h Hexadecimal numbers (lower and upper case) int
x Lower-case hexadecimal numbers int
X Upper-case hexadecimal numbers int

Do remember though that most often a straight type-less {} will suffice where a more complex type specification might have been used.

So, for example, some typed parsing, and None resulting if the typing does not match:

>>> parse('Our {:d} {:w} are...', 'Our 3 weapons are...')
<Result (3, 'weapons') {}>
>>> parse('Our {:d} {:w} are...', 'Our three weapons are...')

And messing about with alignment:

>>> parse('with {:>} herring', 'with     a herring')
<Result ('a',) {}>
>>> parse('spam {:^} spam', 'spam    lovely     spam')
<Result ('lovely',) {}>

Note that the “center” alignment does not test to make sure the value is actually centered. It just strips leading and trailing whitespace.

See also the unit tests at the end of the module for some more examples. Run the tests with “python -m parse”.

Version history (in brief):

  • 1.1.3 type conversion is automatic based on specified field types. Also added “f” and “n” types.
  • 1.1.2 refactored, added compile() and limited from parse import *
  • 1.1.1 documentation improvements
  • 1.1.0 implemented more of the Format Specification Mini-Language and removed the restriction on mixing fixed-position and named fields
  • 1.0.0 initial release

This code is copyright 2011 Inc ( See the end of the source file for the license of use.

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