Trying to understand a very simple class - from the book "dive intopython"

[walterbyrd]

Example 5.6. Coding the FileInfo Class
class FileInfo(UserDict):
"store file metadata"
def __init__(self, filename=None):
UserDict.__init__(self) (1)
self["name"] = filename (2)


What I do not understand is the last line. I thought 'self' was
supposed to refer to an instance of the object. But the square
brackets seem to indicate a list. Except you can not assign values to
a list like that. So where does "name" come from?

I am sure this is totally simple, but I missing something.

 

[Marco Mariani]

> I am sure this is totally simple, but I missing something.

Do you know what a dictionary is?

 

[walterbyrd]

Do you know what a dictionary is?

Yes, but I thought a dictionary used curly brackets. Is the object a
dictionary?

 

[Diez B. Roggisch]

Yes, but I thought a dictionary used curly brackets. Is the object a
dictionary?

foo = {"key" : "i'm the value of a dictionary"}
print foo["key"]

I suggest you read the tutorial:

http://docs.python.org/tutorial/datastructures.html#dictionaries

Diez

 

[Chris Rebert]

Yes, but I thought  a dictionary used curly brackets. Is the object a
dictionary?

Indeed. Note how you're subclassing `UserDict` (its name is similar to
`dict` for a reason).
See the UserDict docs for more info:
http://docs.python.org/library/userdict.html

Cheers,
Chris

 

[Dave Angel]

Example 5.6. Coding the FileInfo Class
class FileInfo(UserDict):
"store file metadata"
def __init__(self, filename=ne):
UserDict.__init__(self) (1)
self["name"] =ilename (2)


What I do not understand is the last line. I thought 'self' was
supposed to refer to an instance of the object. But the square
brackets seem to indicate a list. Except you can not assign values to
a list like that. So where does "name" come from?

I am sure this is totally simple, but I missing something.

You're deriving your class from UserDict, which is a "Class that
simulates a dictionary." So you can use dictionary semantics on
"self." "name" is a key for the dict.