[Python-ideas] Python Isn't Perfect: adding a 'gotchas' section to the tutorial

Greg Ewing greg.ewing at canterbury.ac.nz
Mon Dec 12 06:11:32 CET 2011

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 the *obvious* thing to do. So I wouldn't
class it as a "gotcha" in the same sense as truly Python-specific
features like default argument evaluation and list comprehension
variable scope.

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.

Furthermore, copying some kinds of things automatically and not
others (as some other languages such as VB and Java do) makes the
rules needlessly complicated and difficult to remember.

So Python does the simplest possible thing and doesn't copy
anything by default. If you want a copy, you need to do something
explicit to make it happen.

> Unfortunately it is almost certainly too late to propose fixes
 > (if appropriate) for such quirks in Python 3

Python's assignment behaviour is most definitely *not* something
that needs "fixing"!


More information about the Python-ideas mailing list