Richard, I don't think I can provide you with a "why" for dynamic typing. It's a choice Guido made early on, one that is central to the language, and one that I think pays off in the long run. Values are objects, which are typed. Names are untyped, and can refer to any value, regardless of what value they referred to in the past. Therefore, it doesn't make sense to talk about the type of a name, which is all that a type declaration could do.

This is very different than some other programming languages, but I don't know if it could be called unintuitive. I'm not sure anything about programming could truly be called intuitive, I think the closest we can get is "familiar". Certainly if you've worked with statically typed languages before, then dynamic typing is unfamiliar.

I worry that you are still placing dynamic typing into a category you call "gotchas" or "quirks", with the word "fixes" nearby. Dynamic typing cannot be "fixed", it is central to the language. I think it is great to write something to help those new to Python, but you should be sure that you fully understand Python before you undertake it.

--Ned.

On 12/11/2011 4:30 PM, Richard Prosser wrote:

Ned,

I accept your comment about the tutorial - I wasn't sure about that. Thanks.

However I still feel that there are some aspects of the language which are not in the true spirit of Python (i.e. 'intuitive').

The discussion about default mutable types is one of these. It seems to me that the essential problem is that of assignment in general, which (I believe) creates a reference on the LHS to the object on the RHS, rather than having a copy operation to make the two objects completely separate. That can be confusing in other contexts, not just with default parameters.

If I am to write a 'gotchas' FAQ or whatever then I would like to understand the reasoning behind such design decisions but I can't find any 'deep' explanations at present - just several posts about people being puzzled! A similar comment applies to the lack of type declarations.

So if you or anyone else can explain exactly why such odditties are implemented I would be grateful.

Unfortunately it is almost certainly too late to propose fixes (if appropriate) for such quirks in Python 3 but at least I should be able provide arguments as to why things are done the way they are.

Richard

On 11 December 2011 01:21, <python-ideas-request@python.org> wrote:

Message: 2
Date: Sat, 10 Dec 2011 13:34:15 -0500
From: Ned Batchelder <ned@nedbatchelder.com>
To: Richard Prosser <richard.prosser@mail.com>
Cc: python-ideas@python.org
Subject: Re: [Python-ideas] Python Isn't Perfect: adding a 'gotchas'
section to the tutorial
Message-ID: <4EE3A627.8010002@nedbatchelder.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 12/10/2011 9:16 AM, Richard Prosser wrote:
> Although I love Python there are some aspects of the language design
> which are disappointing and which can even lead to problems in some cases.
>
> A classic example is a mutable default argument having the potential
> to produce unexpected side-effects, as a consequence of the
> non-intuitive scoping rules.
>
> Another awkward 'feature' is the requirement for a trailing comma in
> singleton tuples, due I believe to the use of expression parentheses
> rather than (say) the use of special brackets like chevrons.
>
> Something that I personally wish for is the ability to declare
> variable types 'up front' but that facility is missing from Python.
>
One of these things is not like the others. Mutable default arguments,
and singleton tuples are surprises that make sense once you understand
things on a deeper level. It makes sense to call them out as a
"gotcha": a common stumbling block for learners. But "no type
declarations" is not a wart, it's a fundamental feature of the language
that is immediately apparent from the first lesson.

--Ned.
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