View Full Version : Dive into Python example error ?

July 8th, 2008, 09:43 AM
I am learning python by reading Dive into Python (http://www.diveintopython.org) by Mark Pilgrim. I was reading chapter 5 - Objects and Object Orientation (http://www.diveintopython.org/object_oriented_framework/index.html) and I am kind of stuck with example 5.11 (http://www.diveintopython.org/object_oriented_framework/userdict.html) and example 5.12 (http://www.diveintopython.org/object_oriented_framework/special_class_methods.html).

According to the examples FileInfo class has been defined as :

class FileInfo(dict):
"store file metadata"
def __init__(self, filename=None):
self["name"] = filename

and then the following function is added :

def __getitem__(self, key): return self.data[key]

But am I missing something here ? I ran the code & to confirm my initial doubts - it gives an error because FileInfo does not contain a data member called data. Is this a mistake or have I made an error somewhere ?

I tried rewriting the FileInfo class (up until that section only) as follows :

class FileInfo(dict):
"Display file metadata"
def __init__(self, filename=None):
self.data = {}
self.data["name"] = filename
def __getitem__(self, key = None):
if key is not None:
return self.data[key]
return self.data

What is not working is if I try just instantiating an object of FileInfo type say 'test' and just hitting test in the interpreter - it returns and empty dictionary. Is that how the functionality is supposed to be when you define the __getitem__ sepecial function ? Shouldn't it return the entire dictionary if I just enter 'test' in the interpreter ?

Rohit Arondekar

July 8th, 2008, 10:49 AM
i would recommend learning Python from the official Python tutorial. Dive into Python is quite dated.

July 8th, 2008, 10:54 AM
I don't think __getitem__() gets called if you simply type the name in the interpreter:

>>> t = {'a': 1, 'b': 2}
>>> t.__str__()
"{'a': 1, 'b': 2}"
>>> t.__repr__()
"{'a': 1, 'b': 2}"
>>> t.__getitem__('a')
>>> t.__getitem__()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __getitem__() takes exactly one argument (0 given)

Maybe you need to check __repr__?

July 8th, 2008, 12:55 PM
rikxik ~ That does shed some light.

I actually liked Dive into Python - however if there are mistakes then I guess I will have to switch over to the official tutorial and docs. So the main question is - is the example wrong ? Because if it is then shouldn't someone notify the author to correct the mistake ?

July 8th, 2008, 04:18 PM
is the example wrong ? Because if it is then shouldn't someone notify the author to correct the mistake ?

if it looks wrong, and maybe it's **you** who should notify author? :-)

Seems like you are experienced programmer (just new to Python). Try Guido's tutorial. Also, you may compare explanations in free e-books linked in wiki in my sig (even if wiki is more for beginners).

July 8th, 2008, 04:19 PM
Actually you don't need __getitem__ if you are extending dict.

#! /usr/bin/env python

class FileInfo(dict):
"store file metadata"
def __init__(self, filename=None):
self["name"] = filename

#def __getitem__(self, key):
# return self[key]

if __name__ == "__main__":
f = FileInfo("foo")
print f
print f["name"]
f["genre"] = 31
print f


{'name': 'foo'}
{'genre': 31, 'name': 'foo'}

July 8th, 2008, 04:32 PM
pmasiar ~ I am not an experienced programmer actually rather a beginner. So I just wanted to confirm that it was a mistake in the example.

I'll send him a mail now.

Thanks for all the help and pmasiar, your wiki is awesome I've already bookmarked it. :)

July 8th, 2008, 06:17 PM
If you are beginner, don't dive: learn to swim first! :-)

With python (unlike Java), you don't even have to start with objects and OOP until well past beginner stage. Use system object but don't worry about creating your own - use dicts and be happy.