Writing a parser in python

Simple top-Down, parsing in Python


writing a parser in python

Writing, simple, parser in Python, ivanovo

Building your Own Custom Parser by hand. You may need to pick the second option if you have particular needs. Both in the sense that the language you need to parse cannot be parsed with traditional parser generators, or you have specific requirements that you cannot satisfy using a typical parser generator. For instance, because you need the best possible performance or a deep integration between different components. A tool Or Library to generate a parser. In all other cases the third option should be the default one, because is the one that is most flexible and has the shorter development time. That is why on this article we concentrate on the tools and libraries that correspond to this option.

Writing, files in Python

I like having these classes english because it adds a nice structure to the parse tree. From pyparsing import * class pnode(object "Base class for parser elements" def _init self, tokens super(PNode, self)._init kens tokens def _str self return u" ".join(map(lambda x: unicode(x kens) word def _repr self return self._str # Target classes class Integer(PNode def _init self, tokens super(Integer, self)._init tokens). This is an article similar to a previous one we wrote: Parsing in java, so the introduction is the same. Skip to chapter 3 if you have already read. If you need to parse a language, or document, from Python there are fundamentally three ways to solve the problem: use an existing library supporting that specific language: for example a library to parse xml building your own custom parser by hand a tool. Use An Existing Library, the first option is the best for well known and supported languages, like xml or html. A good library usually include also api to programmatically build and modify documents in that language. This is typically more of what you get from a basic parser. The problem is that such libraries are not so common and they support only the most common languages. In other cases you are out of luck.

Its docs are quite detailed and thorough, and full of examples. Having gone through this tutorial, you should easily digest them without feeling overwhelmed. This is my base pattern for writing a parser in Python by using the pyparsing library. It is slightly more complicated than a hello world in pyparsing, but I think it is more useful as a small example of writing a parser for a real grammar. A base class pnode is used to provide utility functions to classes implementing parse tree nodes,. Turning a parse tree into the original string (except all whitespace is replaced by single space). It assumes that tokens in the input where separated review by whitespace, and that all whitespace is the same. For a particular grammar, i use python classes to represent nodes in the parse tree; these classes get created by calling the setParseAction method on the corresponding bnf element.

writing a parser in python

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Y, answer) else: print " ".format(args. Y, answer) Our program is now simpler, and weve task lost some functionality for the sake of demonstration. Anyways, heres the output: python python 4 2 -q 16 python 4 2 -v 4 to the power 2 equals 16 python 4 2 -vq usage: -h -v -q x y dom : error: argument -q/-quiet: not allowed with argument -v/-verbose python 4 2 -v -quiet. Ive added that last output so you can see the sort of flexibility you get,. Mixing long form options with short form ones. Before we conclude, you probably want to tell your users the main purpose of your program, just in case they dont know: import argparse parser x to the power of y group d_mutually_exclusive_group d_argument -v "-verbose action"store_true d_argument -q "-quiet action"store_true d_argument x typeint, help"the. Y, answer) Note that slight difference in the usage text. Note the -v -q, which tells us that we can either use -v or -q, but not both at the same time: python -help usage: -h -v -q x y calculate x to the power of Y positional arguments: x the base y the exponent.

The following example instead uses verbosity level to display more text instead: import argparse parser gumentParser d_argument x typeint, help"the base d_argument y typeint, help"the exponent d_argument -v "-verbosity action"count default0) args rse_args answer args. Y if rbosity 2: print "Running. Format file if rbosity 1: print " ".format(args. Y print answer Output: python python 4 2 -v 42 16 python 4 2 -vv running 42 16 Conflicting options so far, we have been working with two methods of an gumentParser instance. Lets introduce a third one, it allows for us to specify options that conflict with each other. Lets also change the rest of the program so that the new functionality makes more sense: well introduce the -quiet option, which will be the opposite of the -verbose one: import argparse parser gumentParser group d_mutually_exclusive_group d_argument -v "-verbose action"store_true d_argument -q "-quiet action"store_true d_argument. Quiet: print answer elif rbose: print " to the power equals ".format(args.

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writing a parser in python

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That is, we want any value 2 to be as verbose as possible. Third output not so good. Lets fix that writing bug: import argparse parser gumentParser d_argument square typeint, help"display a square of a given number d_argument -v "-verbosity action"count default0, help"increase output verbosity args rse_args answer args. Square, answer) else: print answer weve just introduced yet another keyword, default. Weve set it to 0 in order to make it comparable to the other int values. Remember that by default, if an optional argument isnt specified, it gets the none value, and that cannot be compared to an int value (hence the typeError essay exception). And: python 4 16 you can go quite far just with what weve learned so far, and we have only scratched the surface.

The argparse module is very powerful, and well explore a bit more of it before we end this tutorial. Getting a little more advanced What if we wanted to expand our tiny program to perform other powers, not just squares: import argparse parser gumentParser d_argument x typeint, help"the base d_argument y typeint, help"the exponent d_argument -v "-verbosity action"count default0) args rse_args answer args. Y if rbosity 2: print " to the power equals ".format(args. Y, answer) elif rbosity 1: print " ".format(args. Y, answer) else: print answer Output: python usage: -h -v x y : error: the following arguments are required: x, y python -h usage: -h -v x y positional arguments: x the base y the exponent optional arguments: -h, -help show this help message and.

Square, answer) else: print answer we have introduced another action, count, to count the number of occurrences of a specific optional arguments: python 4 16 python 4 -v 42 16 python 4 -vv the square of 4 equals 16 python 4 -verbosity -verbosity the square. That should explain the complaint. It also behaves similar to store_true action. Now heres a demonstration of what the count action gives. Youve probably seen this sort of usage before.


And, just like the store_true action, if you dont specify the -v flag, that flag is considered to have none value. As should be expected, specifying the long form of the flag, we should get the same output. Sadly, our help output isnt very informative on the new ability our script has acquired, but that can always be fixed by improving the documentation for our script (e.g. Via the help keyword argument). That last output exposes a bug in our program. Lets fix: import argparse parser gumentParser d_argument square typeint, help"display a square of a given number d_argument -v "-verbosity action"count help"increase output verbosity args rse_args answer args. Square*2 # bugfix: replace with if rbosity 2: print "the square of equals ".format(args. Square, answer) else: print answer And this is what it gives: python 4 -vvv the square of 4 equals 16 python 4 -vvvv the square of 4 equals 16 python 4 Traceback (most recent call last file line 11, in module if rbosity 2: TypeError.

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How about we give this program of ours back the ability to have multiple verbosity values, and actually get to use them: import argparse parser gumentParser d_argument square typeint, help"display a square of a given number d_argument -v "-verbosity typeint, help"increase output verbosity args rse_args. Square*2 if rbosity 2: print "the square of equals ".format(args. Square, answer) elif rbosity 1: print "2 ".format(args. Square, answer) else: print answer And the output: python 4 16 python 4 -v usage: -h -v verbosity square : error: argument -v/-verbosity: expected one argument python 4 -v python 4 -v 2 the square of 4 equals 16 python 4 -v 3 16 These. Lets fix it by restricting the values the -verbosity option can accept: import argparse parser gumentParser d_argument square typeint, help"display a square of a given number d_argument -v "-verbosity typeint, choices0, 1, 2, help"increase output verbosity args rse_args answer args. Square, answer) else: print answer And the output: python 4 -v 3 usage: -h -v 0,1,2 square : error: argument -v/-verbosity: invalid choice: 3 (choose from 0, 1, 2) python 4 -h usage: -h -v 0,1,2 square positional arguments: square display a square. Now, lets use a different approach of playing essay with verbosity, which is pretty common. It also matches the way the cpython executable handles its own verbosity argument (check the output of python -help import argparse parser gumentParser d_argument square typeint, help"display the square of a given number d_argument -v "-verbosity action"count help"increase output verbosity args rse_args answer args.

writing a parser in python

The -help option, which can also be shortened to -h, is the only resume option we get for free (i.e. No need to specify it). Specifying anything else results in an error. But even then, we do get a useful usage message, also for free. Combining Positional and Optional arguments Our program keeps growing in complexity: import argparse parser gumentParser d_argument square typeint, help"display a square of a given number d_argument -v "-verbose action"store_true help"increase output verbosity args rse_args answer args. Square*2 if rbose: print "the square of equals ".format(args. Square, answer) else: print answer And now the output: python usage: -h -v square : error: the following arguments are required: square python 4 16 python 4 -verbose the square of 4 equals 16 python -verbose 4 the square of 4 equals 16 weve brought. Note that the order does not matter.

to change behaviour of the program. In our example, we display more info for each file instead of just showing the file names. The -l in that case is known as an optional argument. Thats a snippet of the help text. Its very useful in that you can come across a program you have never used before, and can figure out how it works simply by reading its help text. The basics Let us start with a very simple example which does (almost) nothing: import argparse parser gumentParser rse_args following is a result of running the code: python python -help usage: -h optional arguments: -h, -help show this help message and exit python -verbose usage. The second one starts to display the usefulness of the argparse module. We have done almost nothing, but already we get a nice help message.

List information about the files (the current warming directory by default). Sort entries alphabetically if none of -cftuvsux nor -sort is specified. A few concepts we can learn from the four commands: The ls command is useful when run without any options at all. It defaults to displaying the contents of the current directory. If we want beyond what it provides by default, we tell it a bit more. In this case, we want it to display a different directory, pypy. What we did is specify what is known as a positional argument. Its named so because the program should know what to do with the value, solely based on where it appears on the command line.

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This tutorial is intended to be a gentle introduction to argparse, the recommended command-line parsing module in the python standard library. This was written for argparse in Python. A few details are different.x, especially some exception messages, which were improved.x. Note, there are two other modules that fulfill the same task, namely getopt (an equivalent for getopt from the c language) and the deprecated optparse. Note also that argparse is based on optparse, and therefore very similar in terms of usage. Concepts, lets show the sort of functionality that we are going to explore in this introductory tutorial by making use of the ls command: ls cpython devguide pypy tch ls pypy ctypes_configure demo dotviewer include lib_pypy lib-python. ls -l total 20 drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython drwxr-xr-x 4 wena wena 4096 Feb 8 12:04 devguide -rwxr-xr-x 1 wena wena 535 Feb 19 00:05 drwxr-xr-x 14 wena wena 4096 Feb 7 00:59 pypy -rw-r-r- 1 wena wena 741 Feb.


Writing a parser in python
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To create a parser you should provide a sequence of tokens and. This tutorial is intended to be a gentle introduction to argparse, the recommende d command-line parsing module in the python standard library.

3 Comment

  1. This is my base pattern for writing a parser in Python by using th e pyparsing library. It is slightly more complicated than a hello world. A recursive descent parser generator.

  2. In Simple Iterator-based Parsing, i described a way to write simple recursive-descent parsers in Python, by passing around the current. For simple grammars I really like parsing Expression Grammars (PEGs which amount to a disciplined, structured way of writing. From time to time one might need to write simple language parser t o implement some domain specific language for his application. This is a good overview using parsimonius Writing Simple parser in Python This is the python list of language parsers LanguageParsing - python wiki.

  3. We are also concentrating on one target language: Python. This als o means that (usually) the parser itself will be written in Python. Fredrik lundh july 2008.

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