Grammars and Actions

bootpeg is designed to translate input sequences to arbitrary runtime objects. A parser defines how to match and transform input, which is realised in two separate stages:

• the Grammar matches the input to logical clauses, and

• the Actions transform the matched clauses to runtime objects.

For example, translating the input string "1/2" to the object Fraction(1, 2) requires a grammar clause to match the input and an action to transform it:

fraction:
    | num=integer '/' denom=integer { Fraction(.num, .denom) }
#     ^- - Parser match clause - -^  ^-- transform Action --^

bootpeg handles both separately: The Parser can be automatically created from the grammar using bootpeg itself, while the Actions are defined using regular code and simply provided to bootpeg. This allows grammars to be optimised automatically, and Actions to be changed without affecting the grammar.

Grammar Definition

A Parser can be automatically created from a grammar written in any meta-grammar supported by bootpeg. Based on the grammar, the Parser encodes how to match input to Actions.

>>> from bootpeg.grammars import bpeg
>>> grammar = """
... integer:
...     | ( "0" - "9" )+ { Integer(.*) }
... fraction:
...     | num=integer ' '* '/' ' '* denom=integer { Fraction(.num, .denom) }
... rational:
...     | fraction | integer
... top:
...     | rational (',' ' '* rational)* { .* }
... """
>>> parser = bpeg.parse(grammar)

All bootpeg.grammar.<name>.parse methods default to the Actions required to create a Parser.

Actions Definition

The Actions are Python callables which receive part of the matched input to create the desired output. A set of bootpeg.Actions specifies:

names

A mapping from names to callables and objects, which can be used in grammar actions.

post

A single callable to apply a final transformation of the top-most action result.

These will frequently be classes, though functions and constants are valid as well.

>>> from bootpeg import Actions
>>> from fractions import Fraction
>>> rational_actions = Actions(
...     names={'Integer': int, 'Fraction': Fraction},
...     post=list,
... )