Parser/Composer library for Python
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page "pyPEG – a PEG Parser-Interpreter in Python" {
h1 id=intro > Introduction
p >>
∑Python is a nice ∫scripting language∫. It even gives you access to its
own ∑parser and ∑compiler. It also gives you access to different other
parsers for special purposes like ∑XML and string templates.
>>
p >>
But sometimes you may want to have your own parser. This is what's
ƒpyPEG for. And ƒpyPEG supports ∑Unicode.
>>
p >>
ƒpyPEG is a plain and simple intrinsic parser interpreter framework for
Python version 2.7 and 3.x. It is based on ∫Parsing Expression Grammar∫,
PEG. With ƒpyPEG you can parse many formal languages in a very easy
way. How does that work?
>>
h2 id=installation > Installation
p >>
You can install a «2.x» series ƒpyPEG release from
¬https://pypi.python.org/pypi/pyPEG2 PyPY¬ with:
>>
Code ||
pip install pypeg2
||
h2 id=parsing > Parsing text with pyPEG
p >>
PEG is something like ∫Regular Expressions∫ with recursion. The
grammars are like templates. Let's make an example. Let's say, you
want to parse a function declaration in a C like language. Such a
function declaration consists of:
>>
table style="margin-bottom:3ex;" {
tr {
td red >     
td style="padding-left:.5em;" > type declaration
}
tr {
td orange >     
td style="padding-left:.5em;" > name
}
tr {
td green >     
td style="padding-left:.5em;" > parameters
}
tr {
td blue >     
td style="padding-left:.5em;" > block with instructions
}
}
pre {
code | `red > int` `orange > f`(`green > int a, long b`)
code blue ||
{
do_this;
do_that;
}
||
}
p >>
With ƒpyPEG you're declaring a Python class for each object type you want
to parse. This class is then instanciated for each parsed object. This class
gets an attribute «grammar» with a description what should be parsed in
what way. In our simple example, we are supporting two different things
declared as keywords in our language: «int» and «long». So we're
writing a class declaration for the typing, which supports an «Enum» of
the two possible keywords as its «grammar»:
>>
Code ||
class Type(Keyword):
grammar = Enum( K("int"), K("long") )
||
p >>
Common parsing tasks are included in the ƒpyPEG framework. In this
example, we're using the «Keyword» class because the result will be a
keyword, and we're using «Keyword» objects (with the abbreviation «K»),
because what we parse will be one of the enlisted keywords.
>>
p >>
The total result will be a «Function». So we're declaring a «Function»
class:
>>
Code ||
class Function:
grammar = `red > Type`, …
||
p >>
The next thing will be the name of the «Function» to parse. Names are
somewhat special in ƒpyPEG. But they're easy to handle: to parse a
name, there is a ready made «name()» function you can call in your grammar to
generate a «.name» «Attribute»:
>>
Code ||
class Function:
grammar = `red > Type`, `orange > name()`, …
||
p >>
Now for the «Parameters» part. First let's declare a class for the parameters.
«Parameters» has to be a collection, because there may be many of
them. ƒpyPEG has some ready made collections. For the case of the «Parameters»,
the «Namespace» collection will fit. It provides indexed access by name, and
«Parameters» have names (in our example: «a» and «b»). We write it like this:
>>
Code
||
class Parameters(Namespace):
grammar = …
||
p >>
A single «Parameter» has a structure itself. It has a «Type» and a «name()».
So let's define:
>>
Code
||
class Parameter:
grammar = Type, name()
class Parameters(Namespace):
grammar = …
||
p >>
ƒpyPEG will instantiate the «Parameter» class for each parsed parameter.
Where will the «Type» go to? The «name()» function will generate a
«.name» «Attribute», but the «Type» object? Well, let's move it to an
«Attribute», too, named «.typing». To generate an «Attribute», ƒpyPEG
offers the «attr()» function:
>>
Code
||
class Parameter:
grammar = attr("typing", Type), name()
class Parameters(Namespace):
grammar = …
||
p >>
By the way: «name()» is just a shortcut for «attr("name", Symbol)». It generates
a «Symbol».
>>
p >>
How can we fill our «Namespace» collection named «Parameters»? Well, we have
to declare, how a list of «Parameter» objects will look like in our source text.
An easy way is offered by ƒpyPEG with the cardinality functions. In this case
we can use «maybe_some()». This function represents the asterisk cardinality, *
>>
Code
||
class Parameter:
grammar = attr("typing", Type), name()
class Parameters(Namespace):
grammar = Parameter, maybe_some(",", Parameter)
||
p >>
This is how we express a comma separated list. Because this task is so common,
there is a shortcut generator function again, «csl()». The code below will do
the same as the code above:
>>
Code
||
class Parameter:
grammar = attr("typing", Type), name()
class Parameters(Namespace):
grammar = csl(Parameter)
||
p >>
Maybe a function has no parameters. This is a case we have to consider.
What should happen then? In our example, then the «Parameters» «Namespace» should
be empty. We're using another cardinality function for that case, «optional()». It
represents the question mark cardinality, ?
>>
Code
||
class Parameter:
grammar = attr("typing", Type), name()
class Parameters(Namespace):
grammar = optional(csl(Parameter))
||
p >>
We can continue with our «Function» class. The «Parameters» will be
in parantheses, we just put that into the «grammar»:
>>
Code ||
class Function:
grammar = `red > Type`, `orange > name()`, "(", `green > Parameters`, ")", …
||
p >>
Now for the block of instructions. We could declare another collection for the
Instructions. But the function itself can be seen as a list of instructions. So
let us declare it this way. First we make the «Function» class itself a «List»:
>>
Code ||
class Function(`blue > List`):
grammar = `red > Type`, `orange > name()`, "(", `green > Parameters`, ")", …
||
p >>
If a class is a «List», ƒpyPEG will put everything inside this list,
which will be parsed and does not generate an «Attribute». So with that
modification, our «Parameters» now will be put into that List, too. And
so will be the «Type». This is an option, but in our example, it is not
what we want. So let's move them to an «Attribute» «.typing» and an
«Attribute» «.parms» respectively:
>>
Code ||
class Function(`blue > List`):
grammar = `red > attr("typing", Type)`, `orange > name()`, \\
"(", `green > attr("parms", Parameters)`, ")", …
||
p >>
Now we can define what a «block» will look like, and put it just behind into
the «grammar» of a «Function». The «Instruction» class we have plain and simple.
Of course, in a real world example, it can be pretty complex ;-) Here we just
have it as a «word». A «word» is a predefined «RegEx»; it is «re.compile(r"\w+")».
>>
Code
||
class Instruction(str):
grammar = word, ";"
block = `blue > "{", maybe_some(Instruction), "}"`
||
p >>
Now let's put that to the tail of our «Function.grammar»:
>>
Code ||
class Function(`blue > List`):
grammar = `red > attr("typing", Type)`, `orange > name()`, \\
"(", `green > attr("parms", Parameters)`, ")", `blue > block`
||
p >>
ƒCaveat: pyPEG 2.x is written for Python 3. You can use it with
Python 2.7 with the following import (you don't need that for Python 3):
>>
Code | from __future__ import unicode_literals, print_function
p >>
Well, that looks pretty good now. Let's try it out using the «parse()» function:
>>
Code
||
>>> from pypeg2 import *
>>> class Type(Keyword):
... grammar = Enum( K("int"), K("long") )
...
>>> class Parameter:
... grammar = attr("typing", Type), name()
...
>>> class Parameters(Namespace):
... grammar = optional(csl(Parameter))
...
>>> class Instruction(str):
... grammar = word, ";"
...
>>> block = "{", maybe_some(Instruction), "}"
>>> class Function(List):
... grammar = attr("typing", Type), name(), \\
... "(", attr("parms", Parameters), ")", block
...
>>> f = parse("int f(int a, long b) { do_this; do_that; }",
... Function)
>>> f.name
Symbol('f')
>>> f.typing
Symbol('int')
>>> f.parms["b"].typing
Symbol('long')
>>> f[0]
'do_this'
>>> f[1]
'do_that'
||
h2 id=composing > Composing text
p >>
ƒpyPEG can do more. It is not only a framework for parsing text, it can
compose source code, too. A ƒpyPEG «grammar» is not only “just like” a
template, it can actually be used as a template for composing text.
Just call the «compose()» function:
>>
Code
||
>>> compose(f, autoblank=False)
'intf(inta, longb){do_this;do_that;}'
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p >>
As you can see, for composing first there is a lack of whitespace. This
is because we used the automated whitespace removing functionality of
ƒpyPEG while parsing (which is enabled by default) but we disabled the
automated adding of blanks if violating syntax otherwise. To improve on
that we have to extend our «grammar» templates a little bit. For that
case, there are callback function objects in ƒpyPEG. They're only
executed by «compose()» and ignored by «parse()». And as usual, there
are predefined ones for the common cases. Let's try that out. First
let's add «blank» between things which should be separated:
>>
Code
||
class Parameter:
grammar = attr("typing", Type), ◊blank◊, name()
class Function(List):
grammar = attr("typing", Type), ◊blank◊, name(), \\
"(", attr("parms", Parameters), ")", block
||
p >>
After resetting everything, this will lead to the output:
>>
Code ||
>>> compose(f, autoblank=False)
'int◊ ◊f(int◊ ◊a, long◊ ◊b){do_this;do_that;}'
||
p >>
The «blank» after the comma `code { "int a," mark " "; "long b"}` was
generated by the «csl()» function; «csl(Parameter)» generates:
>>
Code | Parameter, maybe_some(",", blank, Parameter)
h3 id=indenting > Indenting text
p >>
In C like languages (like our example) we like to indent blocks.
Indention is something, which is relative to a current position. If
something is inside a block already, and should be indented, it has to
be indented two times (and so on). For that case ƒpyPEG has an indention
system.
>>
p >>
The indention system basically is using the generating function «indent()»
and the callback function object «endl». With indent we can mark what should
be indented, sending «endl» means here should start the next line of the
source code being output. We can use this for our «block»:
>>
Code
||
class Instruction(str):
grammar = word, ";", ◊endl◊
block = "{", ◊endl◊, maybe_some(◊indent(◊Instruction◊)◊), "}", ◊endl◊
class Function(List):
grammar = attr("typing", Type), blank, name(), \\
"(", attr("parms", Parameters), ")", ◊endl◊, block
||
p >>
This changes the output to:
>>
Code ||
>>> print(compose(f))
int f(int a, long b)
{
do_this;
do_that;
}
||
h3 id=usercallbacks > User defined Callback Functions
p >>
With User defined Callback Functions ƒpyPEG offers the needed flexibility
to be useful as a general purpose template system for code generation. In
our simple example let's say we want to have processing information in
comments in the «Function» declaration, i.e. the indention level in a comment
bevor each «Instruction». For that we can define our own Callback Function:
>>
Code {
| class Instruction(str):
mark
||
def heading(self, parser):
return "/* on level " + str(parser.indention_level) \\
+ " */", endl
||
}
p >>
Such a Callback Function is called with two arguments. The first
argument is the object to output. The second argument is the parser
object to get state information of the composing process. Because this
fits the convention for Python methods, you can write it as a method of
the class where it belongs to.
>>
p >>
The return value of such a Callback Function must be the resulting text.
In our example, a C comment shell be generated with notes. We can put
this now into the «grammar».
>>
Code
||
class Instruction(str):
def heading(self, parser):
return "/* on level " + str(parser.indention_level) \\
+ " */", endl
grammar = ◊heading◊, word, ";", endl
||
p >>
The result is corresponding:
>>
Code
||
>>> print(compose(f))
int f(int a, long b)
{
/* on level 1 */
do_this;
/* on level 1 */
do_that;
}
||
h2 id=xmlout > XML output
p >>
Sometimes you want to process what you parsed with
¬http://www.w3.org/TR/xml/ the XML toolchain¬, or with
¬http://fdik.org/yml the YML toolchain¬. Because of that, ƒpyPEG has an
XML backend. Just call the «thing2xml()» function to get «bytes» with
encoded XML:
>>
Code
||
>>> from pypeg2.xmlast import thing2xml
>>> print(◊thing2xml(f, pretty=True)◊.decode())
<Function typing="int" name="f">
<Parameters>
<Parameter typing="int" name="a"/>
<Parameter typing="long" name="b"/>
</Parameters>
<Instruction>do_this</Instruction>
<Instruction>do_that</Instruction>
</Function>
||
p >>
The complete sample code
¬http://fdik.org/pyPEG2/sample1.py you can download here¬.
>>
div id="bottom" {
"Want to download? Go to the "
a "#top", "^Top^"; " and look to the right ;-)"
}
}