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simple parsing library

Project description

Simple parsing library for Python.

There’s not much documentation yet, and the performance is probably pretty bad, but if you want to give it a try, go for it!

Feel free to send me your feedback at Or use the github issue tracker.


To install sourcer:

pip install sourcer

If pip is not installed, use easy_install:

easy_install sourcer

Or download the source from github and install with:

python install


Example 1: Hello, World!

Let’s parse the string “Hello, World!” (just to make sure the basics work):

from sourcer import *

# Let's parse strings like "Hello, foo!", and just keep the "foo" part.
greeting = 'Hello' >> Opt(',') >> ' ' >> Pattern(r'\w+') << '!'

# Let's try it on the string "Hello, World!"
person1 = parse(greeting, 'Hello, World!')
assert person1 == 'World'

# Now let's try omitting the comma, since we made it optional (with "Opt").
person2 = parse(greeting, 'Hello Chief!')
assert person2 == 'Chief'

Some notes about this example:

  • The >> operator means “Discard the result from the left operand. Just return the result from the right operand.”
  • The << operator similarly means “Just return the result from the result from the left operand and discard the result from the right operand.”
  • Opt means “This term is optional. Parse it if it’s there, otherwise just keep going.”
  • Pattern means “Parse strings that match this regular expression.”

Example 2: Parsing Arithmetic Expressions

Here’s a quick example showing how to use operator precedence parsing:

from sourcer import *

Int = Pattern(r'\d+') * int
Parens = '(' >> ForwardRef(lambda: Expr) << ')'
Expr = OperatorPrecedence(
    Int | Parens,
    Prefix('+', '-'),
    InfixLeft('*', '/'),
    InfixLeft('+', '-'),

# Now let's try parsing an expression.
t1 = parse(Expr, '1+2^3/4')
assert t1 == Operation(1, '+', Operation(Operation(2, '^', 3), '/', 4))

# Let's try putting some parentheses in the next one.
t2 = parse(Expr, '1*(2+3)')
assert t2 == Operation(1, '*', Operation(2, '+', 3))

# Finally, let's try using a unary operator in our expression.
t3 = parse(Expr, '-1*2')
assert t3 == Operation(Operation(None, '-', 1), '*', 2)

Some notes about this example:

  • The * operator means “Take the result from the left operand and then apply the function on the right.”
  • In this case, the function is simply int.
  • So in our example, the Int rule matches any string of digit characters and produces the corresponding int value.
  • So the Parens rule in our example parses an expression in parentheses, discarding the parentheses.
  • The ForwardRef term is necessary because the Parens rule wants to refer to the Expr rule, but Expr hasn’t been defined by that point.
  • The OperatorPrecedence rule constructs the operator precedence table. It parses operations and returns Operation objects.

Example 3: Building an Abstract Syntax Tree

Let’s try building a simple AST for the lambda calculus. We can use Struct classes to define the AST and the parser at the same time:

from sourcer import *

class Identifier(Struct):
    def parse(self): = Word

class Abstraction(Struct):
    def parse(self):
        self.parameter = '\\' >> Word
        self.body = '. ' >> Expr

class Application(LeftAssoc):
    def parse(self):
        self.left = Operand
        self.operator = ' '
        self.right = Operand

Word = Pattern(r'\w+')
Parens = '(' >> ForwardRef(lambda: Expr) << ')'
Operand = Parens | Abstraction | Identifier
Expr = Application | Operand

t1 = parse(Expr, r'(\x. x) y')
assert isinstance(t1, Application)
assert isinstance(t1.left, Abstraction)
assert isinstance(t1.right, Identifier)
assert t1.left.parameter == 'x'
assert == 'x'
assert == 'y'

t2 = parse(Expr, 'x y z')
assert isinstance(t2, Application)
assert isinstance(t2.left, Application)
assert isinstance(t2.right, Identifier)
assert == 'x'
assert == 'y'
assert == 'z'

Example 4: Tokenizing

It’s often useful to tokenize your input before parsing it. Let’s create a tokenizer for the lambda calculus.

from sourcer import *

class LambdaTokens(TokenSyntax):
    def __init__(self):
        self.Word = r'\w+'
        self.Symbol = AnyChar(r'(\.)')
        self.Space = Skip(r'\s+')

# Run the tokenizer on a lambda term with a bunch of random whitespace.
Tokens = LambdaTokens()
ans1 = tokenize(Tokens, '\n (   x  y\n\t) ')

# Assert that we didn't get any space tokens.
assert len(ans1) == 4
(t1, t2, t3, t4) = ans1
assert isinstance(t1, Tokens.Symbol) and t1.content == '('
assert isinstance(t2, Tokens.Word) and t2.content == 'x'
assert isinstance(t3, Tokens.Word) and t3.content == 'y'
assert isinstance(t4, Tokens.Symbol) and t4.content == ')'

# Let's use the tokenizer with a simple grammar, just to show how that
# works.
Sentence = Some(Tokens.Word) << '.'
ans2 = tokenize_and_parse(Tokens, Sentence, 'This is a test.')

# Assert that we got a list of Word tokens.
assert all(isinstance(i, Tokens.Word) for i in ans2)

# Assert that the tokens have the expected content.
contents = [i.content for i in ans2]
assert contents == ['This', 'is', 'a', 'test']

In this example, the Skip term tells the tokenizer that we want to ignore whitespace. The AnyChar term tell the tokenizer that a symbol can be any one of the characters (, \, ., ). Alternatively, we could have used:

Symbol = r'[(\\.)]'

Example 5: Parsing Significant Indentation

We can use sourcer to parse languages with significant indentation. Here’s a bare-bones example to demonstrate one possible approach.

from sourcer import *

class TestTokens(TokenSyntax):
    def __init__(self):
        # Let's just use words, newlines, and spaces in this example.
        self.Word = r'\w+'
        self.Newline = r'\n'
        # In this case, we'll say that an indent is a newline followed by
        # some spaces, followed by a word.
        self.Indent = r'(?<=\n) +(?=\w)'
        # And let's just throw out all other space characters.
        self.Space = Skip(' +')

# All our token classes are attributes of this ``Tokens`` object. It's
# essentially a namespace for our token classes.
Tokens = TestTokens()

class InlineStatement(Struct):
    def parse(self):
        # Let's say an inline-statement is just some word tokens. We'll use
        # ``Content`` to get the string content of each token (since in this
        # case, we don't care about the tokens themselves).
        self.words = Some(Content(Tokens.Word))

    def __repr__(self):
        # We'll define a ``repr`` method so that we can easily check the
        # parse results. We'll just put a semicolon after each statement.
        return '%s;' % ' '.join(self.words)

class Block(Struct):
    def parse(self, indent=''):
        # A block is a bunch of statements at the same indentation,
        # all separated by some newline tokens.
        self.statements = Statement(indent) // Some(Tokens.Newline)

    def __repr__(self):
        # In this case, we'll put a space between each statement and enclose
        # the whole block in curly braces. This will make it easy for us to
        # tell if our parse results look right.
        return '{%s}' % ' '.join(repr(i) for i in self.statements)

def Statement(indent):
    # Let's say there are two ways to get a statement:
    # - Get an inline-statement with the current indentation.
    # - Get a block that is indented farther than the current indentation.
    return (CurrentIndent(indent) >> InlineStatement
        | IncreaseIndent(indent) ** Block)

def CurrentIndent(indent):
    # The point of this function is to return a parsing expression that
    # matches the current indent (which is provided as an argument).
    return Return('') if indent == '' else indent

def IncreaseIndent(current):
    # To see if the next indentation is more than the current indentation,
    # we peek at the next token, using ``Expect``, and we get its string
    # content using ``Content``. The ``^`` operator means "require". In this
    # case, we require that the next indentation is longer than the current
    # indentation.
    token = Expect(Content(Tokens.Indent))
    return token ^ (lambda token: len(current) < len(token))

# Let's say that a program is a block, optionally surrounded by newlines.
# (The ``>>`` and ``<<`` operators discard the newlines in this case.)
OptNewlines = List(Tokens.Newline)
Program = OptNewlines >> Block << OptNewlines

test = '''
print foo
while true
    print bar
    if baz
        then break

# Let's parse the test case and then use ``repr`` to make sure that we get
# back what we expect.
ans = tokenize_and_parse(Tokens, Program, test)
expect = '{print foo; while true; {print bar; if baz; {then break;}} exit;}'
assert repr(ans) == expect

More Examples

Parsing Excel formula and some corresponding test cases.

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