Naming convention in Collections: why are some lowercase and others CapWords?

Question

Why the mixture of lowercase and UpperCamelCase?

namedtuple
deque   
Counter 
OrderedDict
defaultdict

Why collections instead of Collections?

I sometimes do this for example:

from collections import default_dict

by mistake. What rule of thumb can I use to avoid such mistakes in the future?

Answer 1

The collections module follows the PEP 8 Style Guide:

Modules should have short, all-lowercase names.

This is why it's collections

---

Almost without exception, class names use the CapWords convention.

This is why it's Counter and OrderedDict, because they are both classes:

>>> collections.Counter
<class 'collections.Counter'>
>>> collections.OrderedDict
<class 'collections.OrderedDict'>

---

namedtuple is a function, so it does not follow the style guide mentioned above. deque and defaultdicts are types, so they also do not:

>>> collections.deque
<type 'collections.deque'>
>>> collections.namedtuple
<function namedtuple at 0x10070f140>
>>> collections.defaultdict
<type 'collections.defaultdict'>

---

Note: With Python 3.5, defaultdict and deque are now classes too:

>>> import collections
>>> collections.Counter
<class 'collections.Counter'>
>>> collections.OrderedDict
<class 'collections.OrderedDict'>
>>> collections.defaultdict
<class 'collections.defaultdict'>
>>> collections.deque
<class 'collections.deque'>

I assume they kept defaultdict and deque lowercase for backwards compatibility. I would not imagine they'd make such a drastic name change for the sake of a style guide.