functions vs methods

Ben Finney ben+python at benfinney.id.au
Sun Jul 22 04:01:40 EDT 2018


Sharan Basappa <sharan.basappa at gmail.com> writes:

> Is there a difference between functions and methods in Python.

Python's documentation includes a useful Glossary. See the terms
<URL:https://docs.python.org/3/glossary.html#term-method>
<URL:https://docs.python.org/3/glossary.html#term-function>.

Every method is a function; but there are functions that are not
methods.

What distinguishes a method is that it is associated with a specific
class. A method is always a method *of* some class or object.

> For example, this is the text from tutorialpoint on Python:
> Python includes the following list functions - cmp, len etc.

The functions ‘cmp’, ‘len’, are not associated with any particular
class. They can be called without being bound to any object.

> Python includes following list methods - append, count

That means the functions it is referring to are each methods of ‘list’.
Any instance of ‘list’ has methods ‘append’ and ‘count’, bound to that
instance.

> In the first case, len is a function that python provides to which
> list can be passed and in the second case, append is a method within
> list class?

Yes, that's correct.

> If my interpretation is correct, why not make len also as a part of
> list class itself?

Because ‘len’ works with *any* sequence, not only lists. To implement it
as a method of each sequence type, it would have to be implemented on
each type separately, which is a design that is needlessly more complex.

This is common in Python: it uses so-called “duck typing”, where the way
an object behaves is more important than its type. Because “what is the
length of this object” is a question valid for a broad variety of types,
the design decision was made to allow it to accept any type for which
that query makes sense.

<URL:https://docs.python.org/3/glossary.html#term-duck-typing>

Your particular question is itself a FAQ
<URL:https://docs.python.org/3/faq/design.html#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list>.

-- 
 \           “All progress has resulted from people who took unpopular |
  `\                                      positions.” —Adlai Stevenson |
_o__)                                                                  |
Ben Finney



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