Question

What methods must be implemented in a class definition so that it can be considered as a sequence type and an iterable type? And if there’s a formal definition for them can you also provide a reference to PEP or another document where it is defined?

Answer 1

From the glossary:

An iterable is:

A object with an __iter__() or __getitem__() method. Examples of
iterables include all sequence types (such as list, str, and tuple)
and some non-sequence types like dict and file. When an iterable
object is passed as an argument to the builtin function iter(), it
returns an iterator for the object. This iterator is good for one pass
over the set of values.

An iterator is:

An iterable which has a next() method. Iterators are required to have
an __iter__() method that returns the iterator object itself. An
iterator is good for one pass over the set of values.

A sequence is:

An iterable which supports efficient element access using integer
indices via the __getitem__() special method and defines a len()
method that returns the length of the sequence. Some built-in sequence
types are list, str, tuple, and unicode. Note that dict also supports
__getitem__() and __len__(), but is considered a mapping rather
than a sequence because the lookups use arbitrary immutable keys
rather than integers.