What does a &= b mean?
This depends on the implementation for the types of a and b, but semantics are essentially that
a &= b
is the update of a with its “AND” for b. That “AND” operation could be a set intersection, a binary AND operation, or some other operation that can be implemented by a programmer.
So there are multiple ways of implementing it.
In the following sections, I demonstrate __and__ and __iand__ and give multiple examples of types for which there are different semantics below (set, in which the a object is mutated in-place, and frozenset and int which are immutable, so the variable a is now pointing to the new object).
Explanation
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.
Built-in Example, Sets:
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).
Unimplemented Built-in Example
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.)
Built-in Example, Binary Flags:
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.