On 2011-12-11, at 22:30 , Richard Prosser wrote:

However I still feel that there are some aspects of the language which are not in the true spirit of Python (i.e. 'intuitive'). I have to note that, as far as I know, "intuitive" is not quite part of the "true spirit of Python". The Zen even has a stanza on the subject:

Although that way may not be obvious at first unless you're Dutch.

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. I don't see this as a problem, it's the normal semantics of languages using reference-values for object types.

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. I don't understand, why do you consider dynamic typing to be "an oddity"? There is nothing odd about it, and it's in fact older than computer science itself.

The default arguments issue is an unfortunate interaction of Python's core reference-value semantics and default arguments being implemented as attributes to the function object, evaluated when said function object is created (the feature itself is as old as Python, according to the logs it was added for the 1.0.2 release back in 1994, the changelog seems to peg it to April 14, so your only chance for an explanation of why it was implemented with these semantics is hoping Guido has a very, very good memory. Numerous post-hoc rationalization exist, but only him may hold the true reason).

Unfortunately it is almost certainly too late to propose fixes (if appropriate) for such quirks in Python 3 Most of these have been core semantic attributes of the language for almost two decades now, so even if you had proposed these changes during the Python 3 design cycle I think it's very unlikely they'd have passed: they don't just change Python, they create a very different language with a syntax similar to Python's.

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, mailto:python-ideas-request@python.org> wrote:

Message: 2 Date: Sat, 10 Dec 2011 13:34:15 -0500 From: Ned Batchelder mailto:ned@nedbatchelder.com> To: Richard Prosser mailto:richard.prosser@mail.com> Cc: python-ideas@python.org mailto: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 mailto: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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On Sun, 11 Dec 2011 19:34:07 -0500 Ned Batchelder wrote:

Richard, I don't think I can provide you with a "why" for dynamic typing.

And this is the wrong place to ask. Dynamic typing and naming objects dates back to the precursors to LISP in the mid 50s. You should be asking the people who made that decision. By the same token, have you asked anyone why C/Java/etc. have static typing and name locations? It's an equally valid question. http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

Richard Prosser wrote:

...

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, I would like to understand the reasoning behind such design decisions but I can't find any 'deep' explanations at present So if you or anyone else can explain exactly why such odditties are implemented I would be grateful.

Python's assignment semantics are only an "oddity" to people whose prior exposure to programming languages is very limited. To anyone familiar with almost any other dynamic language -- such as Lisp, Scheme, Smalltalk, or Javascript -- it's not only unsurprising It's not even a question of "dynamic languages", Java and C# reference types have exactly the same semantics (assignment copies

On 2011-12-12, at 06:11 , Greg Ewing wrote: the value part of a type, which is the reference itself). The only people I'd see confused by this are those with significant C++ experience, where assignment of references does indeed go through a copy of the object itself.

As for rationale, it comes down to something like this: Copying large chunks of data is expensive, so it makes sense to do it only when you really need to. And experience shows that most of the time you *don't* need to copy things. Of course technically that copy could very well be performed "on write". This would significantly complexify the runtime as well.

The only people I'd see confused by this are those with significant> C++ experience, where assignment of references does indeed go through> a copy of the object itself.

ಠ_ಠ On Mon, Dec 12, 2011 at 7:59 PM, Masklinn wrote:

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Richard Prosser wrote:

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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, I would like to understand the reasoning behind such design decisions but I can't find any 'deep' explanations at present So if you or anyone else can explain exactly why such odditties are implemented I would be grateful.

Python's assignment semantics are only an "oddity" to people whose prior exposure to programming languages is very limited. To anyone familiar with almost any other dynamic language -- such as Lisp, Scheme, Smalltalk, or Javascript -- it's not only unsurprising It's not even a question of "dynamic languages", Java and C# reference types have exactly the same semantics (assignment copies

On 2011-12-12, at 06:11 , Greg Ewing wrote: the value part of a type, which is the reference itself).

The only people I'd see confused by this are those with significant C++ experience, where assignment of references does indeed go through a copy of the object itself.

...

As for rationale, it comes down to something like this: Copying large chunks of data is expensive, so it makes sense to do it only when you really need to. And experience shows that most of the time you *don't* need to copy things. Of course technically that copy could very well be performed "on write". This would significantly complexify the runtime as well.

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-- ಠ_ಠ

On 2011-12-12, at 17:40 , Terry Reedy wrote:

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By the same token, have you asked anyone why C/Java/etc. have static typing and name locations? It's an equally valid question. C was designed for writing a computing machine operating system with mutable sequential memory slots numbered from 0 to 2**n - 1. On the other hand, there are very few typed assemblies. The question of typing is somewhat orthogonal to that of naming value objects versus storage locations. There are strong stactic typed named value languages like ML. I know, hence my quoting the part about typing, not the part about variable semantics.

They even require different operators for int and float arithmetic, just like assembler. That's got nothing to do with the typing or binding disciplines though, it's an issue of ML's type system. Haskell does not have that issue and is arguably more statically typed than MLs.

Assemblers also type data versus address registers and may have signed versus unsigned int operations. But most typing has to be done by the programmer Which is the point I was trying to make, a point which kind-of goes against the justification used for C being statically typed, as assemblies are also designed for driving systems with mutable sequential memory slots.

On Mon, 12 Dec 2011 11:40:03 -0500 Terry Reedy wrote:

On 12/12/2011 4:00 AM, Masklinn wrote:

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On 2011-12-12, at 09:55 , Terry Reedy wrote:

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On 12/12/2011 12:59 AM, Mike Meyer wrote:

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By the same token, have you asked anyone why C/Java/etc. have static typing and name locations? It's an equally valid question. C was designed for writing a computing machine operating system with mutable sequential memory slots numbered from 0 to 2**n - 1. On the other hand, there are very few typed assemblies. The question of typing is somewhat orthogonal to that of naming value objects versus storage locations. There are strong stactic typed named value languages like ML. They even require different operators for int and float arithmetic, just like assembler.

Yup. You can also find dynamically typed named locations languages, like BCPL (though "untyped" might be more descriptive). It also has different operators for int and float arithmetic, because the locations don't have type information, so it has to come from the operator.