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Date: Wed, 11 Sep 2013 22:41:50 +0200
From: Markus Rother <python@markusrother.de>
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To: python-list@python.org
Subject: Re: Language design
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Hello all,

Thanks for this thread.  Here are my two cents...

On 10.09.2013 08:09, Steven D'Aprano wrote:
> What design mistakes, traps or gotchas do you think Python has? Gotchas 
> are not necessarily a bad thing, there may be good reasons for it, but 
> they're surprising.

    """
    1. Occasionally, one encounters a strange idiom.  Syntactically
    legal and consistent, though:

    >>> +4++4
    8
    >>> -4+-4
    -8
    >>> -4-+4
    -8


    2. Reduce removed from standard library.  That is a big fail, in
    my opinion.


    3. The default return value of methods is None instead of self.
    If it was self, it would be possible to chain method calls (which
    is called a cascade in smalltalk).


    >>> lst = []
    >>> lst.append(1).append(2).append(3) ## FAIL
    Traceback (most recent call last):
    ...
    AttributeError: 'NoneType' object has no attribute 'append'


    Instead, this works:


    >>> class Coll(list):
    ...
    ...     def add(self, e):
    ...         self.append(e)
    ...         return self
    ...
    >>> lst = Coll()
    >>> lst.add(1).add(2).add(3) ## OK
    [1, 2, 3]


    A very practical use case is, that the return value of None makes
    all of these methods unusable for reduce.


    >>> from functools import reduce
    ...
    >>> many = [{1: 'a'}, {2: 'b'}, {3: 'c'}]
    ...
    >>> reduce(lambda d, other : d.update(other), many, {}) ## FAIL
    Traceback (most recent call last):
    ...
    AttributeError: 'NoneType' object has no attribute 'update'


    Again, one would have to define an update function with an
    explicit return value.

    >>> many = [{1: 'a'}, {2: 'b'}, {3: 'c'}]
    ...
    >>> def fn(d, other):
    ...     d.update(other)
    ...     return d
    ...
    >>> reduce(fn, many, {}) ## OK
    {1: 'a', 2: 'b', 3: 'c'}


    4. As has been mentioned already, some built-in functions do magic
    stuff behind the scenes:


    >>> () == []
    False


    But:


    >>> bool(().__eq__([]))
    True


    Because:


    >>> ().__eq__([])
    NotImplemented


    which yields True when cast to boolean.
    """

Greets,
Markus