What types of attributes does the dir function give in python?

Question

I read somewhere that everything in python is an object. I thought, how about the number 2? If I type "dir(2)" into the python interpreter, I get the following output:

dir(2) ['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__', '__delattr__', '__div__', '__divmod__', '__doc__', '__float__', '__floordiv__', '__format__', '__getattribute__', '__getnewargs__', '__hash__', '__hex__', '__index__', '__init__', '__int__', '__invert__', '__long__', '__lshift__', '__mod__', '__mul__', '__neg__', '__new__', '__nonzero__', '__oct__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'imag', 'numerator', 'real']

Evidently the number 2 is an object with these attributes. I get several names of attributes the number 2, like __add__ and conjugate. But if I try 2.conjugate(), I get an error

2.conjugate() SyntaxError: invalid syntax

However dir(2j) has conjugate as a method and I do not get an error when I use the method conjugate.

2j.conjugate() -2j

and

2.0.conjugate() 2.0

also gives me no problems.

More weird things, 2.__add__() and 2.__add__ gives an error even though it is an attribute of 2. I think add refers to the + operation. So why does it show up as __add__ in the list of attributes and not as +? Is dir(2) the list of variables and methods of 2 or does it list other things? What is the difference between __thing__ and thing? When can you call a method the regular way, like object.method() and when do you have to do funky things like 2+2 instead of 2.add(2)?

shx2 · Accepted Answer · 2015-10-15 00:48:15Z

1

conjugate is a method of int, which can be applied to int objects.

To demonstrate, all of these work:

x = 2; x.conjugate()
(2).conjugate()
2 .conjugate()  # note the whitespace

Since the python parser has special handlings of numeric literals, it doesn't allow this awkward syntax:

 2.conjugate()

As explained by @ignacio, this is because the parser sees 2. and treats it as a float, i.e. like (2.)conjugate(), which is not a valid syntax.

However, other than using it with this syntax, the operation is allowed, as demonstrated above.

As for __add__, the __add__ method allows types to define/override how the + opertor applies to them. When you do a+b, the interpreter invokes either a.__add__(b) or b.__add__(a) (the mechanism is more complex than that).

Ordinarily, you shouldn't have to call __add__ directly.

edited Oct 15, 2015 at 0:48

answered Oct 15, 2015 at 0:43

shx2

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Ignacio Vazquez-Abrams · Accepted Answer · 2015-10-15 00:42:46Z

0

The parser sees the "." and thinks that a float is expected.

>>> (2).conjugate
<built-in method conjugate of int object at 0x2242140>

For the rest:

Python Language Reference, Data Model section

answered Oct 15, 2015 at 0:42

Ignacio Vazquez-Abrams

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1 Comment

holdenweb Over a year ago

Technically speaking it's the lexer rather than the parser that makes the lower-level decisions about what constitutes a source "token". Since 2. is a valid token, the parser never gets to see the period.

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