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parse·PyPI

Installation 
pip install parse
Usage

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

parse() is the opposite of format()

The module is set up to only export parse(), search(), findall(), and with_pattern() when import * is used:

>>> from parse import *

From there it’s a simple thing to parse a string:

>>> parse("It's {}, I love it!", "It's spam, I love it!")
<Result ('spam',) {}>
>>> _[0]
'spam'

Or to search a string for some pattern:

>>> search('Age: {:d}\n', 'Name: Rufus\nAge: 42\nColor: red\n')
<Result (42,) {}>

Or find all the occurrences of some pattern in a string:

>>> ''.join(r[0] for r in findall(">{}<", "<p>the <b>bold</b> text</p>"))
'the bold text'

If you’re going to use the same pattern to match lots of strings you can compile it once:

>>> from parse import compile
>>> 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',) {}>

(“compile” is not exported for import * usage as it would override the built-in compile() function)

The default behaviour is to match strings case insensitively. You may match with case by specifying case_sensitive=True:

>>> parse('SPAM', 'spam', case_sensitive=True) is None
True
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 valid Python identifiers, including dotted names; element indexes imply dictionaries (see below for example).

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 {} {}", "The knights who say Ni!")
>>> print(r)
<Result ('knights', 'say', 'Ni!') {}>
>>> print(r.fixed)
('knights', 'say', 'Ni!')
>>> print(r[0])
knights
>>> print(r[1:])
('say', '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'}
>>> print(r['item'])
hand grenade
>>> 'item' in r
True

Note that in only works if you have named fields.

Dotted names and indexes are possible with some limits. Only word identifiers are supported (ie. no numeric indexes) and the application must make additional sense of the result:

>>> r = parse("Mmm, {food.type}, I love it!", "Mmm, spam, I love it!")
>>> print(r)
<Result () {'food.type': 'spam'}>
>>> print(r.named)
{'food.type': 'spam'}
>>> print(r['food.type'])
spam
>>> r = parse("My quest is {quest[name]}", "My quest is to seek the holy grail!")
>>> print(r)
<Result () {'quest': {'name': 'to seek the holy grail!'}}>
>>> print(r['quest'])
{'name': 'to seek the holy grail!'}
>>> print(r['quest']['name'])
to seek the holy grail!

If the text you’re matching has braces in it you can match those by including a double-brace {{ or }} in your format string, just like format() does.

Format Specification

Most often a straight format-less {} will suffice where a more complex format specification might have been used.

Most of format()’s Format Specification Mini-Language is supported:

[[fill]align][sign][0][width][.precision][type]

The differences between parse() and format() are:

Type

Characters Matched

Output

l

Letters (ASCII)

str

w

Letters, numbers and underscore

str

W

Not letters, numbers 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

%

Percentage (converted to value/100.0)

float

f

Fixed-point numbers

float

F

Decimal numbers

Decimal

e

Floating-point numbers with exponent e.g. 1.1e-10, NAN (all case insensitive)

float

g

General number format (either d, f or e)

float

b

Binary numbers

int

o

Octal numbers

int

x

Hexadecimal numbers (lower and upper case)

int

ti

ISO 8601 format date/time e.g. 1972-01-20T10:21:36Z (“T” and “Z” optional)

datetime

te

RFC2822 e-mail format date/time e.g. Mon, 20 Jan 1972 10:21:36 +1000

datetime

tg

Global (day/month) format date/time e.g. 20/1/1972 10:21:36 AM +1:00

datetime

ta

US (month/day) format date/time e.g. 1/20/1972 10:21:36 PM +10:30

datetime

tc

ctime() format date/time e.g. Sun Sep 16 01:03:52 1973

datetime

th

HTTP log format date/time e.g. 21/Nov/2011:00:07:11 +0000

datetime

ts

Linux system log format date/time e.g. Nov 9 03:37:44

datetime

tt

Time e.g. 10:21:36 PM -5:30

time

The type can also be a datetime format string, following the 1989 C standard format codes, e.g. %Y-%m-%d. Depending on the directives contained in the format string, parsed output may be an instance of datetime.datetime, datetime.time, or datetime.date.

>>> parse("{:%Y-%m-%d %H:%M:%S}", "2023-11-23 12:56:47")
<Result (datetime.datetime(2023, 11, 23, 12, 56, 47),) {}>
>>> parse("{:%H:%M}", "10:26")
<Result (datetime.time(10, 26),) {}>
>>> parse("{:%Y/%m/%d}", "2023/11/25")
<Result (datetime.date(2023, 11, 25),) {}>

Some examples of typed parsing with None returned 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...')
>>> parse('Meet at {:tg}', 'Meet at 1/2/2011 11:00 PM')
<Result (datetime.datetime(2011, 2, 1, 23, 0),) {}>

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 centered - it just strips leading and trailing whitespace.

Width and precision may be used to restrict the size of matched text from the input. Width specifies a minimum size and precision specifies a maximum. For example:

>>> parse('{:.2}{:.2}', 'look')           # specifying precision
<Result ('lo', 'ok') {}>
>>> parse('{:4}{:4}', 'look at that')     # specifying width
<Result ('look', 'at that') {}>
>>> parse('{:4}{:.4}', 'look at that')    # specifying both
<Result ('look at ', 'that') {}>
>>> parse('{:2d}{:2d}', '0440')           # parsing two contiguous numbers
<Result (4, 40) {}>

Some notes for the special date and time types:

Note: attempting to match too many datetime fields in a single parse() will currently result in a resource allocation issue. A TooManyFields exception will be raised in this instance. The current limit is about 15. It is hoped that this limit will be removed one day.

Result and Match Objects

The result of a parse() and search() operation is either None (no match), a Result instance or a Match instance if evaluate_result is False.

The Result instance has three attributes:

fixed

A tuple of the fixed-position, anonymous fields extracted from the input.

named

A dictionary of the named fields extracted from the input.

spans

A dictionary mapping the names and fixed position indices matched to a 2-tuple slice range of where the match occurred in the input. The span does not include any stripped padding (alignment or width).

The Match instance has one method:

evaluate_result()

Generates and returns a Result instance for this Match object.

Custom Type Conversions

If you wish to have matched fields automatically converted to your own type you may pass in a dictionary of type conversion information to parse() and compile().

The converter will be passed the field string matched. Whatever it returns will be substituted in the Result instance for that field.

Your custom type conversions may override the builtin types if you supply one with the same identifier:

>>> def shouty(string):
...    return string.upper()
...
>>> parse('{:shouty} world', 'hello world', {"shouty": shouty})
<Result ('HELLO',) {}>

If the type converter has the optional pattern attribute, it is used as regular expression for better pattern matching (instead of the default one):

>>> def parse_number(text):
...    return int(text)
>>> parse_number.pattern = r'\d+'
>>> parse('Answer: {number:Number}', 'Answer: 42', {"Number": parse_number})
<Result () {'number': 42}>
>>> _ = parse('Answer: {:Number}', 'Answer: Alice', {"Number": parse_number})
>>> assert _ is None, "MISMATCH"

You can also use the with_pattern(pattern) decorator to add this information to a type converter function:

>>> from parse import with_pattern
>>> @with_pattern(r'\d+')
... def parse_number(text):
...    return int(text)
>>> parse('Answer: {number:Number}', 'Answer: 42', {"Number": parse_number})
<Result () {'number': 42}>

A more complete example of a custom type might be:

>>> yesno_mapping = {
...     "yes":  True,   "no":    False,
...     "on":   True,   "off":   False,
...     "true": True,   "false": False,
... }
>>> @with_pattern(r"|".join(yesno_mapping))
... def parse_yesno(text):
...     return yesno_mapping[text.lower()]

If the type converter pattern uses regex-grouping (with parenthesis), you should indicate this by using the optional regex_group_count parameter in the with_pattern() decorator:

>>> @with_pattern(r'((\d+))', regex_group_count=2)
... def parse_number2(text):
...    return int(text)
>>> parse('Answer: {:Number2} {:Number2}', 'Answer: 42 43', {"Number2": parse_number2})
<Result (42, 43) {}>

Otherwise, this may cause parsing problems with unnamed/fixed parameters.

Potential Gotchas

parse() will always match the shortest text necessary (from left to right) to fulfil the parse pattern, so for example:

>>> pattern = '{dir1}/{dir2}'
>>> data = 'root/parent/subdir'
>>> sorted(parse(pattern, data).named.items())
[('dir1', 'root'), ('dir2', 'parent/subdir')]

So, even though {‘dir1’: ‘root/parent’, ‘dir2’: ‘subdir’} would also fit the pattern, the actual match represents the shortest successful match for dir1.

Developers

Want to contribute to parse? Fork the repo to your own GitHub account, and create a pull-request.

git clone git@github.com:r1chardj0n3s/parse.git
git remote rename origin upstream
git remote add origin git@github.com:YOURUSERNAME/parse.git
git checkout -b myfeature

To run the tests locally:

python -m venv .venv
source .venv/bin/activate
pip install -r tests/requirements.txt
pip install -e .
pytest
Changelog

This code is copyright 2012-2021 Richard Jones <richard@python.org> See the end of the source file for the license of use.

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