Is a list an instance of a class?

[Kent Johnson]

In the Python tutorial section 9.1, it says, "the word ``object'' in
Python does not necessarily mean a class instance. Like C++ and
Modula-3, and unlike Smalltalk, not all types in Python are classes: the
basic built-in types like integers and lists are not, and even somewhat
more exotic types like files aren't."

Is this still correct or was it made obsolete by Python 2.2? Lists and
files have __class__ attributes, at least:

>>> [].__class__

>>> f = open('ThreadQueue.py')
>>> f.__class__

<type 'file'>

So do integers and floats, though you have to ask nicely for an int: File "<stdin>", line 1
1.__class__
^
SyntaxError: invalid syntax<type 'float'>

Kent

 

[Dan Bishop]

In the Python tutorial section 9.1, it says, "the word ``object'' in
Python does not necessarily mean a class instance. Like C++ and
Modula-3, and unlike Smalltalk, not all types in Python are classes: the
basic built-in types like integers and lists are not, and even somewhat
more exotic types like files aren't."

Is this still correct or was it made obsolete by Python 2.2? Lists and
files have __class__ attributes, at least:

For all practical purposes, it's obsolete. You can inherit from "non-classes" now.
.... def __repr__(self):
.... return 'I' * self
....

IIIIIIIIIIII

[].__class__

f = open('ThreadQueue.py')
f.__class__

<type 'file'>

So do integers and floats, though you have to ask nicely for an int:File "<stdin>", line 1
1.__class__
^
SyntaxError: invalid syntax<type 'int'>

You don't have to ask *that* nicely.
<type 'int'>

 

[Steven Bethard]

You don't have to ask *that* nicely.

<type 'int'>

Why does this work? Or, perhaps my real question is why *doesn't* it work
without the space? Pointers to the appropriate point in the docs would be
fine... I assume it's something about making parsing easier...?

Thanks,

Steve

 

[Erik Max Francis]

Why does this work? Or, perhaps my real question is why *doesn't* it work
without the space? Pointers to the appropriate point in the docs would be
fine... I assume it's something about making parsing easier...?

It's because otherwise it's confusing it for a floating point literal.
All you need to do is put a break so that the parser can't get confused.
1 ._class__ or (1).__class__ will work just fine.

 

[Alex Martelli]

Why does this work? Or, perhaps my real question is why *doesn't* it work
without the space? Pointers to the appropriate point in the docs would be
fine... I assume it's something about making parsing easier...?

It's to make lexing more predictable: it's known as maximal-much lexing
(without backtracking: backtracking in lexical analysis would really but
really be bad). '1.' (no space) is a token (stands for a float
literal). So, '1.__class__' is two tokens: float '1.' followed by
identified '__class__'.


Alex

 

[Steve Holden]

Why does this work? Or, perhaps my real question is why *doesn't* it work
without the space? Pointers to the appropriate point in the docs would be
fine... I assume it's something about making parsing easier...?

Thanks,

Steve

Yup, it's because a dot immediately following an integer would be parsed
(strictly "lexed", I suppose) as a floating point constant,m aking the
__class__ a syntax error.

regards
Steve

 

[Russell Blau]

Yup, it's because a dot immediately following an integer would be parsed
(strictly "lexed", I suppose) as a floating point constant,m aking the
__class__ a syntax error.

regards
Steve

And, in case anyone was wondering (as I was), the following works, too,
despite the fact that it looks incredibly odd:
<type 'float'>