On Sat, Feb 11, 2017 at 4:32 AM, boB Stepp <robertvst...@gmail.com> wrote:
>
> This bit got me experimenting. Since the integer "5" is an integer
> object instance, I am wondering why I can't do:
>
> py3: 5.__repr__()
> File "<stdin>", line 1
> 5.__repr__()
> ^
> SyntaxError: invalid syntax
>
> , but I can do:
>
> py3: x = 5
> py3: x.__repr__()
> '5'
The parser sees "5." as a floating point number. You can use
parentheses to force it to parse 5 as an integer:
>>> (5).__repr__()
'5'
> But in class examples I've seen posted, I do not recall __repr__ ever
> being defined. So I infer that most of the time I should not or need not
> do so, but under what circumstances should I do so?
Classes inherit a default __repr__ from `object`. For example:
>>> object.__repr__(5)
'<int object at 0xa68ac0>'
Define a custom __repr__ when it's useful for debugging to include
additional information. It also gets called for str() if you don't
implement a custom __str__.
> Another curiosity question. If I type:
>
> py3: repr(int)
> "<class 'int'>"
>
> I get what I expect, but if I do the same with my custom class, "boB",
> I instead get:
>
> py3: repr(boB)
> "<class '__main__.boB'>"
>
> Why the difference between the Python class "int" and my custom class
> "boB"? Did I not define __repr__() correctly?
The __repr__ defined by your class is used for instances of the class,
not for the class itself. In the above you're seeing the return value
from the metaclass __repr__ that's defined by `type`:
In CPython, type.__repr__ is implemented in C as type_repr in
Objects/typeobject.c. If it can determine the module and qualified
name of a class, then it uses the template "<class '%U.%U'>".
Otherwise it uses the template "<class '%s'>" with the basic char
*tp_name from the PyTypeObject.
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