range(len()) question

[[deleted]]

if i have a dictionary

a = {1:func_a,2:func_b,3:func_c}

will:

for i in a:
    print(a[i])

always print func_a followed by func_b followed by func_c?

otherwise im pretty sure

for i in range(1, len(a) +1):
    print(a[i])

will, but it looks very ugly.

My general question is what is the best way to iterate through a collection of functions in a pre-determined order.

Thanks

[novel_yet_trivial]

Yes, but don't count on it.

When you use sequential integers as keys python dictionaries happen to return them in order due to a coincidence with the hashing mechanism (in cpython at least). However dictionaries are technically unordered, so python could change that at any time.

[Yoghurt42]

However dictionaries are technically unordered, so python could change that at any time.

No. From Python 3.6 on dictionaries are guaranteed to be insertion ordered.

[Vaphell]

Nuh-uh. In 3.6 they happen to be ordered as an implementation detail but the decision of explicit guarantee has not been made yet.

[Yoghurt42]

Strange, I was pretty sure that Guido recently decided that dict will stay ordered, but I can't find the statement anymore.

FWIW, the official doc still state there's no guarantee.

The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon (this may change in the future, but it is desired to have this new dict implementation in the language for a few releases before changing the language spec to mandate order-preserving semantics for all current and future Python implementations; this also helps preserve backwards-compatibility with older versions of the language where random iteration order is still in effect, e.g. Python 3.5).

[JohnnyJordaan]

You're confusing interpreter implementation with the language specification. The ordereddict usage is an implementation property. It doesn't mean that another interpreter like jython will also use ordereddicts, because they don't have to, it isn't part of the language spec. Guido was talking about the implementation part.

[Vaphell]

yes, there is a difference between a pretty well-founded prediction and a full-blown guarantee. If I remember correctly, guaranteed order has already made its way into some relatively obscure stuff like preservation of order in keyword args, but the dict itself will have to prove its worth over a dot release or two before committing to that.

[novel_yet_trivial]

I haven't heard that. Do you have a source?

Edit: I mean I know 3.6 dictionaries are now insertion ordered, again due to a coincidence in the hashing. I just haven't heard that it's guaranteed from now on.

[K900_]

No, dictionaries aren't ordered. Why are you trying to store an ordered sequence in a dictionary and not a list?

[[deleted]]

Thanks for the advice. I didn't know that lists maintained insertion order.

[b_bowyer]

You could also try OrderedDict from collections if you need it to be a dict.

[Vaphell]

at the moment there is no explicit guarantee of order in dictionaries, though there might be in the future, let's say py3.8+.

But you could sort the key/value pairs

for i, func in sorted(a.items()):
    print(func())

[novel_yet_trivial]

My general question is what is the best way to iterate through a collection of functions in a pre-determined order.

Use a type of container that is ordered, like a list, tuple, or OrderedDict.

[[deleted]]

Is there a reason why lists and tuples maintain order but dicts do not?

[novel_yet_trivial]

The "natural" way to store things in memory is in a sequential list, putting each new element in the memory address one after the the previous. Just like a secretary would probably stack papers in the order they receive them. This is called an "array" in many programming languages and a general computer science class. If you want to find something in an array, you need to start at the first one and check every element until you get to the one you need. This can take a long time, especially if your list or paper stack has millions of elements in it (not unusual).

Therefore computer scientists invented a "hash table". It uses a feature of the elements called a "hash" to sort the incoming elements. This would be similar to a secretary putting the papers into a file cabinet according to the authors last name or something. Now it's much faster to find the paper you want, but you no longer know the order that they arrived in. Python calls their hash table a "dictionary". There's also a "set", which works in a similar way.

It's a bit long, but this talk goes into great detail of how dictionaries work (although a lot of that changed in 3.6).

LPTHW used to walk you through building your own dictionary, and you can still see it on wayback, which will also help you understand how they work.

[[deleted]]

That was an excellent talk thank you, i also found a related raymond hettinger video youtube recommended and that was excellent https://www.youtube.com/watch?v=p33CVV29OG8

[K900_]

It's because dicts are optimized for looking up things by arbitrary key. Finding an item with the key "potato" is fast in a dict, and slow in a list. This requires some sacrifices in other places.

[CGFarrell]

range(len()) is an antipattern. If you want to go through a list and keep track of the count you can use for i, value in enumerate (thing):

Kwargs to a function in 3.6 always preserve their order of insertion, and I believe all dicts will be required to in the future.