DSL for pattern matching on .py parse trees.
The ptTools packages provides a domain specific language, for pattern matching on trees - python parsetrees in particular.
Matching parsetree nodes instead of regular expression based string search, allows fine-grained, semantic control over search results. The second major benefit is that the matches are instantiated parsetree nodes, that can be manipulated and recompiled. Parsetree based search is of interest especially when manipulating or substituting selected nodes, e.g. for testing, or code analysis.
The domain specific language of ptTools patterns consists of a family of pattern objects, all of which can be expressed in valid python code. Binary and unary operators similar to those of regular expressions are available. The resulting pattern language resembles the grammar rules it matches.
Emphasis was put on an intuitive interface, and a clean and easily extensible object-oriented design. The components are loosely coupled. For example the pattern objects can be matched against tuples as well as against attributed parsetree nodes. The parsetree nodes may or may not hold references to tokens, which again may or may not be attributed.
The object-oriented design of the RegularLanguageAcceptor classes in ptTools.misc.acceptors was inspired by Lukas Renggli’s Petit Parser written in Smalltalk.
A first example - Creating a simple pattern
The following first example matches a for loop in a python source file. We will first create the parsetree as a nested tuple, then create the pattern, and finally we will match the pattern against the parsetree.
Our starting point is the corresponding grammar rule, taken from http://docs.python.org/3.2/reference/grammar.html:for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
Lowercase expressions are non-terminals that can be imported from the symbol module. Strings represent terminals with a string value. Bracketed sub-rules are optional. For the sake of simplicity we disregard the else option in our pattern. Also, we will replace the rules exprlist, testlist and suite by wildcards.
Creating the parse tree
Here, the parse tree is created from a short source segment. The result is a nested tuple of the form:node ::= (INT (, (node|STR))*)>>> import parser ... >>> source = ''' ... for k in range(10): ... print(k) ... ''' >>> root = parser.suite(source).totuple()
Note, that the for_stmt non-terminal seen in the rule above, is not the grammar’s start symbol, but will be nested somewhere in the tuple.
Creating a pattern
Next, we create a pattern that matches any for-statement without the else option. As a universal wildcard pattern, we instantiate an AnyPattern() and assign it to the underscore. The TuplePattern is - as the name suggests - a pattern that matches tuples. It may hold other patterns or literal values.>>> from symbol import for_stmt >>> from ptTools.ptpatterns import TuplePattern as Tup >>> from ptTools.ptpatterns import AnyPattern as Any >>> _ = Any() ... >>> pattern = Tup(for_stmt, 'for', _, 'in', _, ':', _) >>> pattern (296, (1, 'for'), _, (1, 'in'), _, (11, ':'), _)
where the integers originate from the symbol and token modules.>>> import symbol >>> import token >>> 296==symbol.for_stmt and 1==token.NAME and 11==token.COLON True
Repeated for comparison: The rulefor_stmt: 'for' exprlist 'in' testlist ':' suite
is matched by:```TuplePattern(for_stmt, 'for', _, 'in', _, ':', _)```
represented as:```(296, (1, 'for'), _, (1, 'in'), _, (11, ':'), _)```
Note, that string literals in the pattern declaration are replaced by TuplePattern instances matching the appropriate parsetree nodes. If a token exists, whose string value equals the given string, that token’s identifier is inserted, otherwise the identifier defaults to token.NAME. In the above example ':' was replaced by TuplePattern(token.COLON, ':'), because the colon exists as a token. The fact that in and for are keywords, does not mean they do exist as tokens, too.
Matching the pattern
As mentioned earlier, the pattern does not match the entire parsetree.>>> pattern.matches(root) False
It does however match somewhere in the parsetree.>>> pattern.matches_in(root) True >>> match = pattern.first_match_in(root) >>> isinstance(match, tuple) True
We can retrieve the matched node, but formatting was lost during parsing:>>> from ptTools.parsetree import tupletraversal >>> ' '.join([s for s in tupletraversal.leaves(match)]) 'for k in range ( 10 ) : print ( k ) '
Example two - Processing matched nodes
This example demonstrates a slightly more complex pattern description, that matches any class definition in python source. The example also shows how to capture matched regions, and apply functions to subpatterns.
>>> from symbol import classdef >>> from token import NAME >>> import ptTools >>> from ptTools.ptpatterns import TuplePattern as Tup >>> from ptTools.ptpatterns import _ ... >>> pattern = Tup(classdef, 'class', (NAME, _), ['(', [ _ ], ')'], ':', _)
Compare this pattern to the grammar rule definition of classdef taken from http://docs.python.org/3.2/reference/grammar.html:classdef: 'class' NAME ['(' [arglist] ')'] ':' suite
Unless we are interested in the class definition’s body, we may replace the pattern’s tail with another wildcard:>>> more = ~_ ## Creating a Kleene Closure over Any(). ... >>> pattern = Tup(classdef, 'class', (NAME, _), more)
Adding callback functions
To send the matched regions to callback functions, simply append ‘% callable’ to a pattern. For instance, if we would like to print the class name from the above example, we would simply write:```pattern = Tup(classdef, 'class', (NAME, _%print), more)```
Note, that the left hand side of the % operator must have been evaluated to a PatternObject prior to evaluating the entire expression. If we would like to print the entire terminal node, we have to be explicit, and type:```pattern = Tup(classdef, 'class', Tup(NAME, _)%print, more)```
Now, assume, one would like to count classes in a module. For that purpose we define an inc function with the side effect of incrementing a counter, and which is to be called every time our classdef pattern matches. All we have to do is to add a callback to inc to the pattern with the % operator.>>> count = 0 >>> def inc(ignore): ... globals()['count'] += 1 ... >>> pattern = Tup(classdef, 'class', (NAME, _%print), more)%inc
Note, that this does two things: It prints the classname and then increments the counter. The order of the two is determined by the traversal order through our parsetree (which is preorder, and hence lexical order of the source code). In other words, print will be called before inc, because the class name is encountered before the entire class definition ends.
Callback functions can be chained (e.g. ‘%inc%input’), and are executed in the order of appending.
As seen in the previous example, PatternObjects have a .match(node) method, that tries to match a parsetree node, and if successful applies its callback functions to the matched regions. Its return value is None.
We now appply that method to every node in the parsetree, starting at its root. If the pattern matches in a node, all queued callback functions will be called with the matched node as their only argument.>>> cls_lst =  ... >>> pattern = Tup(classdef, 'class', (NAME, _%cls_lst.append), more)%inc >>> pattern (329, (1, 'class'), (1, _), ~(_))>>> root = ptTools.parsetree.fromfile('ptpatterns/ptpatterns.py') >>> root.preorder_do(pattern.match) ... >>> 'TuplePattern' in cls_lst True
Currently, five classes are defined in ptTools.ptpatterns.ptpatterns.>>> count 5
Also, see ptTools.examples:
- ptTools.examples.pycat is a colorizer printing python source code to the command line. Performance is worse than that of e.g. pygmentize. However, it may be a good starting point when designing patterns and visualizing their matches in a source file.
- ptTools.examples.extract demonstrates recompilation of a captured segment.
Both examples are executable.
Please refer to the examples. (Also, see ptTools.misc.acceptors).
- For the complete python grammar see: http://docs.python.org/3.2/reference/grammar.html
- Nonterminals can be found in: /usr/include/python3.2/graminit.h as well as in the module: symbol.
- Reference for terminals is found in the modules: token and tokenize.
This program was published under the GNU GENERAL PUBLIC LICENSE.
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