Pythonic Parsing Programs
The Cover Your Bases Answer
The simplest solution to parsing is to parse a file-like object. Again for the newbies, I say file-like because Python has duck typing. According to Wikipedia this term, duck typing, comes from the saying of an old poet—James Whitcomb Riley:
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
But I prefer the scene in Monty Python and the Holy Grail, when wise Bedemir uses his powers of logic to determine whether witches are made of wood and therefore flammable—fit to be burned:
BEDEMIR: So, how do we tell whether she is made of wood? VILLAGER #1: Build a bridge out of her. ... BEDEMIR: Aah, but can you not also build bridges out of stone? VILLAGER #2: Oh, yeah. BEDEMIR: Does wood sink in water? VILLAGER #1: No, no. VILLAGER #2: It floats! It floats! BEDEMIR: What also floats in water? VILLAGER #1: Bread! VILLAGER #2: Apples! VILLAGER #3: Very small rocks! VILLAGER #1: Cider! VILLAGER #2: Great gravy! VILLAGER #1: Cherries! VILLAGER #2: Mud! VILLAGER #3: Churches -- churches! VILLAGER #2: Lead -- lead! ARTHUR: A duck. CROWD: Oooh. BEDEMIR: Exactly! So, logically..., VILLAGER #1: If... she.. weighs the same as a duck, she's made of wood. BEDEMIR: And therefore--? VILLAGER #1: A witch!
(Courtesy of sacred-texts.com/neu/mphg/mphg.htm)
We can apply the same
principle—duck typing—to file objects. If an object behaves like
a file—has a
method—then it is a file. By having this interface we can easily
deal with file-like objects, but also string data, and filenames as
will be illustrated.
Implementing the Interface
Another interesting note from
the table above is that most of the parsing mechanisms are not tied
to a class. Most are functions that create an object, populate it
from data within the file and return the object (
being the exception as it has no function, though the
module has both a
function and a
method on the
class). In addition isolating parsing from the data adheres to the
Single Responsibility Principle.
The author’s opinion is that
an overloaded function or method (such as
which accepts both files and filenames, or
which accepts both strings or files) is a violation of “Explicit
is better than implicit” and “In the face of ambiguity, refuse
the temptation to guess”. These methods are not taking advantage
of duck typing, but magic typing, going beyond Postel’s
Law. The Python language makes a hard distinction about the
difference between an Iterable and an Iterator. A
similar comparison could be made between a filename, and a file
object. One is a duck and the other is a duck factory.
A Simple Example
The example below parses the
contents of the Unix
file and stores the result in a list within the
from collections import namedtuple User = namedtuple('User', 'name, pw, id, gid, groups, home, shell') class Passwd(object): def __init__(self): self.users =  def parse(fp): pw = Passwd() for line in fp: data = line.strip().split(':') pw.users.append(User(*data)) return pw
A couple of notes regarding the implementation:
parsefunction populates a
Passwdobject has one responsibility, knowing about the users. It is not responsible for parsing
If a user wanted to create a
object from a filename—
# from filename with open('/etc/passwd') as fin: pw = parse(fin)
function nor the
object needs to worry about opening or closing the file.
If the data from the file was
already in memory, utilizing the built-in module
enables parsing from that end:
# from string from StringIO import StringIO as sio fin = sio('''pulse:x:113:121:PulseAudio daemon,,,:/var/run/pulse:/bin/false\nsaned:x:114:123::/home/saned:/bin/false''') pw = parse(fin)
Advantages of File-interfaces
Hopefully this long-winded post has provided you some insight into parsing files in Python. Prior art is found in the standard library and 3rd party modules. If you want to be fancy and all accommodating, you can, but in reality you are probably just adding unnecessary complexity.
To review a parsing method that relies only on a file interface is:
- accepts generators, iterators that are potentially very long, in
addition to files, sockets, and
- Easier to manage - you don’t want to be in the business of handling and closing people’s files
- Usable for users with a filename
- Usable for users with data from a file
Go forth and parse!
Matt Harrison (@__mharrison__) is a Python developer at Fusion-io where he helps build data analysis tools.
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