[Python-Dev] PEP for new dictionary implementation

[Terry Reedy]

On 2/8/2012 2:18 PM, Mark Shannon wrote:

A pretty clear draft PEP.

> Changes to repr() output and iteration order:
> For most cases, this will be unchanged.
> However for some split-table dictionaries the iteration order will
> change.
>
> Neither of these cons should be a problem.
> Modules which meddle with the internals of the dictionary
> implementation are already broken and should be fixed to use the API.

So are modules that depend on set and dict iteration order and the 
consequent representations.

> The iteration order of dictionaries was never defined and has always been
> arbitrary; it is different for Jython and PyPy.

I am pretty sure iteration order has changed between CPython versions in 
the past (and that when it did, people got caught). The documentation 
for doctest has section 25.2.3.6. Warnings. It starts with this very issue!
'''
doctest is serious about requiring exact matches in expected output. If 
even a single character doesn’t match, the test fails. This will 
probably surprise you a few times, as you learn exactly what Python does 
and doesn’t guarantee about output. For example, when printing a dict, 
Python doesn’t guarantee that the key-value pairs will be printed in any 
particular order, so a test like

 >>> foo()
{"Hermione": "hippogryph", "Harry": "broomstick"}
is vulnerable! One workaround is to do

 >>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True
instead. Another is to do

 >>> d = sorted(foo().items())
 >>> d
[('Harry', 'broomstick'), ('Hermione', 'hippogryph')]
'''
(Object addresses and full-precision float representations are also 
discussed.)

-- 
Terry Jan Reedy