This reminds me of the "copy list" expression in Python:

    In [1]: a = [1, 2, 3]
    In [2]: b = a
    In [3]: a is b
    Out[3]: True
    In [4]: b = a[:]
    In [5]: a is b
    Out[5]: False

Usually this kind of "optimization" is done to avoid copying large ammounts of data in memory, but as you said, unless it's widesepread knowledge or explicitly stated, it can lead to some confusing situations/bugs.