Functions, methods, and how many arguments do I have to give them?

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Why do the following lines give me the same result?

str.upper('hello')

and

'hello'.upper()

I tried to do the same with list.append but got a TypeError.

list.append([1]) 

Is the str type in python overloaded? How can this be achieved by writing a class/function? I would appreciate an example.

Thanks,

RP.

list.append takes two arguments - the list to modify and the element to append. So you need to do it like this:

ls = [1]
list.append(ls, 2)

which is equivalent to the much more popular:

ls.append(2)

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str.upper and list.append are both functions.

str.upper takes one argument.

>>> str.upper('test')
'TEST'

list.append takes two arguments.

>>> my_list = []
>>> list.append(my_list, 1)
>>> my_list
[1]

str.upper and list.append (like other functions) are also non-data-descriptors with a __get__ method which in this context has two implications:

  1. When you access the function through the class via the dot notation (str.upper, list.append) the function's __get__ method (i.e. string.upper.__get__ and list.append.__get__) is called but it returns just the function itself.
  2. When you access the function through an instance (my_string.upper, my_list.append) the function's __get__ method is called and it will return a new callable acting like the original function, but with whatever was "in front of the dot" automatically passed as the first argument. .

That's why you need to pass 1 - 1 = 0 arguments when calling my_string.upper() and 2 - 1 = 1 argument when calling my_list.append(1).

>>> 'my_string'.upper()
'MY_STRING'
>>>
>>> my_list = []
>>> my_list.append(1)
>>> my_list
[1]

You could even get these modified callables (methods) by explicitly calling __get__ and passing the argument to be bound (what has been before the dot) as its argument.

>>> my_string = 'my_string'
>>> upper_maker = str.upper.__get__(my_string)
>>> upper_maker()
'MY_STRING'
>>> 
>>> my_list = []
>>> appender = list.append.__get__(my_list)
>>> appender(1)
>>> my_list
[1]

Finally, here's a short example demonstrating how descriptor instances can detect whether they are being accessed via their owner-class or via an instance.

class Descriptor:
    def __get__(self, instance, owner_class):
        if instance is None:
            print('accessed through class')
            # list.append.__get__ would return list.append here
        else:
            print('accessed through instance')
            # list.append.__get__ would build a new callable here
            # that takes one argument x and that internally calls
            # list.append(instance, x)

class Class:
    attribute = Descriptor()

Class.attribute # prints 'accessed through class'

instance = Class()
instance.attribute # prints 'accessed through instance'

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Quoting Dave Kirbys answer from Relationship between string module and str:

There is some overlap between the string module and the str type, mainly for historical reasons. In early versions of Python str objects did not have methods, so all string manipulation was done with functions from the string module. When methods were added to the str type (in Python 1.5?) the functions were left in the string module for compatibility, but now just forward to the equivalent str method.

However the string module also contains constants and functions that are not methods on str, such as formatting, character translation etc.

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There is nothing at all magical going on with str (except that we have a nice syntactic shortcut to creating one using ""). You can write a class that behaves like str and list to see more clearly what is happening here.

class MyClass():
    def __init__(self, arg):
        self.val=str(arg)

    def do_thing(self):
        self.val = "asdf"

    def do_thing_with_arg(self, arg):
        self.val = "asdf " + str(arg)

    def __repr__(self):
        return self.val


my_thing = MyClass("qwerty")

# this is like 'hello'.upper()
my_thing.do_thing()
print(my_thing)
# it prints 'asdf'

my_thing = MyClass("qwerty")

# this is like str.upper('hello')
MyClass.do_thing(my_thing)
print(my_thing)
# it prints 'asdf'

my_thing = MyClass("qwerty")

# this is like my_list.append('qwerty')
my_thing.do_thing_with_arg('zxcv')
print(my_thing)
# it prints 'asdf zxcv'

my_thing = MyClass("qwerty")

# this is like list.append(my_list, 'qwerty')
MyClass.do_thing_with_arg(my_thing, 'zxcv')
print(my_thing)
# it prints 'asdf zxcv'

The short version is, you're invoking what looks like an "instance method" on a class, but you are supplying the instance ('self') yourself as the first argument to the function call.

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Comments
  • at which list you do expect to append 1?
  • What would your list.append([1]) append where?
  • What's the relation between your first example for upper cases and append an integer to a list?
  • "How can this be achieved by writing a class/function?" How can what be achieved? It's not at all clear what you are asking for.
  • This question is only trivial on the surface. +1
  • Wouldn't str.upper and list.append fall under the category of instance method under the str and list class?
  • @Idlehands It's a little word-thinky. If I had to try to be very correct, I would say: str.upper and list.append are functions that return methods when their __get__ method is called with a non-None instance argument - where I would define a method as a callable which partially (i.e. with bound first argument) applies a function defined inside a class.
  • Thanks for the clarification, I was genuinely confused because my background is not in comp-sci so terminologies and paradigms are weak spots for me. My understanding was that any functions defined within a class (i.e. a dot preceding its name) is always a method (instance/class/static) hence my original question. In this case I figured str.upper would have been defined similar as class str: def upper(self): where it either runs as instance.upper() or str.upper(instance).
  • @Idlehand The whole descriptor topic is hard. Rule of thumb when writing code: my_class.function_name gives you a function to which you must pass all arguments. instance_of_class.function_name gives you a method to which instance_of_class is automatically passed as the first argument (usually named self).
  • this is probably the best answer one can give here. Don't understand the -1. Well done.
  • It probably got downvoted because it was literally a straight quote from another answer in a related thread.