semantics of the |= operator

Terry Reedy tjreedy at
Thu Aug 21 23:16:40 CEST 2008

akva wrote:
> Hi All,
> what's the exact semantics of the |= operator in python?
> It seems that a |= d is not always equivalent to a = a | d

The manual explicitly specifies that mutable objects may implement the 
operation part of operation-assignments by updating in place  -- so that 
the object assigned is a mutated version of the original rather than a 
new object.

The value equivalency only applies in the namespace in which the 
statement appears.

> For example let's consider the following code:
> def foo(s):
>    s = s | set([10])
> def bar(s):
>    s |= set([10])
> s = set([1,2])
> foo(s)
> print s # prints set([1, 2])

Put the print inside foo and you will get set([1,2,10]), as with bar.

> bar(s)
> print s # prints set([1, 2, 10])
> So it appears that inside bar function the |= operator modifies the
> value of s in place rather than creates a new value.

This has nothing to do with being inside a function.


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