Attack a sacred Python Cow

Jordan jordanrastrick at gmail.com
Thu Jul 24 07:41:54 CEST 2008


Hi everyone,

I'm a big Python fan who used to be involved semi regularly in
comp.lang.python (lots of lurking, occasional posting) but kind of
trailed off a bit. I just wrote a frustration inspired rant on my
blog, and I thought it was relevant enough as a wider issue to the
Python community to post here for your discussion and consideration.

This is not flamebait. I love Python, and I'm not out to antagonise
the community. I also realise that one of the issues I raise is way
too ingrained to be changed now. I'd just like to share my thinking on
a misstep in Python's guiding principles that has done more harm than
good IMO. So anyway, here's the post.

I've become utterly convinced that at least one criticism leveled at
my favourite overall programming language, Python, is utterly true and
fair. After quite a while away from writing Python code, I started
last night on a whim to knock up some code for a prototype of an idea
I once had. It's going swimmingly; the Python Image Library, which I'd
never used before, seems quick, intuitive, and with the all the
features I need for this project. As for Python itself, well, my heart
still belongs to whitespace delimitation. All the basics of Python
coding are there in my mind like I never stopped using them, or like
I've been programming in this language for 10 years.

Except when it comes to Classes. I added some classes to code that had
previously just been functions, and you know what I did - or rather,
forgot to do? Put in the 'self'. In front of some of the variable
accesses, but more noticably, at the start of *every single method
argument list.* This cannot be any longer blamed as a hangover from
Java - I've written a ton more code, more recently in Python than in
Java or any other OO language. What's more, every time I go back to
Python after a break of more than about a week or so, I start making
this 'mistake' again. The perennial justification for this 'feature'
of the language? That old Python favourite, "Explicit is better than
implicit."

I'm sorry, but EXPLICIT IS NOT NECESSARILY BETTER THAN IMPLICIT.
Assembler is explicit FFS. Intuitive, clever, dependable, expected,
well-designed *implicit* behaviour is one of the chief reasons why I
use a high level language. Implicitly garbage collect old objects for
me? Yes, please!

I was once bitten by a Python wart I felt was bad enough to raise and
spend some effort advocating change for on comp.lang.python (never got
around to doing a PEP; partly laziness, partly young and inexperienced
enough to be intimidated at the thought. Still am, perhaps.)

The following doesn't work as any sane, reasonable person would
expect:

# Blog code, not tested
class A():
  def __eq__(self, obj):
    return True
a = A()
b = []
assert a == b
assert not (a != b)

The second assertion fails. Why? Because coding __eq__, the most
obvious way to make a class have equality based comparisons, buys you
nothing from the != operator. != isn't (by default) a synonym for the
negation of == (unlike in, say, every other language ever); not only
will Python let you make them mean different things, without
documenting this fact - it actively encourages you to do so.

There were a disturbingly high number of people defending this
(including one quite renowned Pythonista, think it might have been
Effbot). Some had the temerity to fall back on "Explicit is better
than implict: if you want != to work, you should damn well code
__ne__!"

Why, for heaven's sake, should I have to, when in 99.99% of use cases
(and of those 0.01% instances quoted in the argument at the time only
one struck me as remotely compelling) every programmer is going to
want __ne__ to be the logical negation of __eq__? Why, dear Python,
are you making me write evil Java-style language power reducing
boilerplate to do the thing you should be doing yourself anyway?
What's more, every programmer is going to unconciously expect it to
work this way, and be as utterly as mystified as me when it fails to
do so. Don't tell me to RTFM and don't tell me to be explicit. I'll
repeat myself - if I wanted to be explicit, I'd be using C and
managing my own memory thank you very much. Better yet, I'd explicitly
and graphically swear - swear in frustration at this entrenched design
philosophy madness that afflicts my favourite language.

I think the real problem with the explicit is better than implicit,
though, is that while you can see the underlying truth its trying to
get at (which is perhaps better expressed by Ruby's more equivocal,
less dependable, but more useful Principle of Least Surprise), in its
stated form its actually kind of meanginless and is used mainly in
defence of warts - no, we'll call them for what they are, a language
design *bugs*.

You see, the problem is, there's no such thing of explict in
programming. Its not a question of not doing things implicitly; its a
question of doing the most sensible thing implicitly. For example this
python code:

some_obj.some_meth(some_arg1, some_arg2)

is implicitly equivalent to

SomeClass.some_meth(some_obj, some_arg1, some_arg2)

which in turn gives us self as a reference to some_obj, and Python's
OO model merrily pretends its the same as Java's when in fact is a
smarter version that just superficially looks the same.

The problem is that the explicit requirement to have self at the start
of every method is something that should be shipped off to the
implicit category. You should have to be explicit, yes - explicit when
you want the *other* behaviour, of self *not* being an argument,
because thats the more unusual, less likely case.

Likewise,

a != b

is implicitly equivalent to something like calling this function (may
not be correct, its a while since I was heavily involved in this
issue):

def equal(a, b):
  if hasattr(a, "__ne__"): return a.__ne__(b)
  if hasattr(b, "__ne__"): return b.__ne__(a)
  if hasattr(a, "__cmp__"): return not (a.__cmp__(b) == 0)
  if hasattr(b, "__cmp__"): return not (b.__cmp__(a) == 0)
  return not (a is b)

There's absolutely nothing explicit about this. I wasn't arguing for
making behaviour implicit; I was arguing for changing the stupid
implict behaviour to something more sensible and less surprising.

The sad thing is there are plenty of smart Python programmers who will
justify all kinds of idiocy in the name of their holy crusade against
the implict.

If there was one change I could make to Python, it would be to get
that damn line out of the Zen.



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