Why do some built-in constructors begin with a lower case letter?

Question

Why do some constructors like int(), list(), set() and many others starts with a lower case letter instead of upper case? Shouldn't this be written Int(), List(), Set() and so on?

Answer 1

These types were original factory functions, not types. As such they got a lower-case name:

$ python1.5
Python 1.5.2 (#1, Apr  1 2009, 22:55:54)  [GCC 4.1.2 20070925 (Red Hat
4.1.2-27)] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> type(int)
<type 'builtin_function_or_method'>

That they are now types anyway is a historical artefact

All built-in types now follow this convention, including set and frozenset, which were added after the type unification that made int et al types.

Answer 2

Despite the historical reason for most of these, builtin names that abide to Pythons' naming conventions don't use CapWords. This is specified in PEP 8:

Note that there is a separate convention for builtin names: most builtin names are single words (or two words run together), with the CapWords convention used only for exception names and builtin constants.