What does the &=
operator mean in Python, and can you give me a working example?
I am trying to understand the __iand__
operator.
I just don't know what &=
means and have looked online but couldn't find it.
Understandable that you can't find much reference on it. I find it hard to get references on this too, but they exist.
The i
in iand
means in-place, so it's the in-place operator for &
. &=
calls the __iand__
operator, if it is implemented. If not implemented, it is the same as x = x & y
.
It's primarily used to update the intersection of built-in set types:
>>> a = set('abc')
>>> a &= set('cbe')
>>> a
set(['c', 'b'])
which is the same as:
>>> a = set('abc')
>>> a.__iand__(set('cbe'))
set(['c', 'b'])
It is very similar to calling the set.intersection_update
method, and would be used in control flow as you would do an in-place update of any object or variable (if the object is immutable).
The less commonly used immutable frozenset object would be replaced in memory on the inplace update, and the variable name would point to the new object in memory.
>>> a = frozenset('abc')
>>> a &= set('bce')
>>> a
frozenset({'c', 'b'})
In this case, since frozenset doesn't implement an __iand__
method,
>>> a = frozenset('abc')
>>> a.__iand__(set('cbe'))
Traceback (most recent call last):
File "<pyshell#160>", line 1, in <module>
a = frozenset('abc'); a.__iand__(set('cbe'))
AttributeError: 'frozenset' object has no attribute '__iand__'
it is (*nearly) identical to
a = a & set('bce')
*(I say nearly because if you examine the bytecode, you'll see that the underlying implementation treats sets and frozensets the same, even though frozensets don't have __iand__
, and sets do, because each calls INPLACE_AND
, at least for compiled functions.)
Similar to Sets, we can use the &=
to update the intersection of binary option flags where the value for True
is 1
. Below, we demonstrate that the "binary AND", (akin to intersection) of the binary numbers 1110
and 1011
is 1010
:
>>> option_flags = int('1110', 2)
>>> option_flags
14
>>> option_flags &= int('1011', 2)
>>> option_flags
10
>>> bin(option_flags)
'0b1010'
Since int
objects are not mutable, like the frozenset
example, this actually only reassigns the variable option_flags
to the newly calculated value.