Question

I’m learning Python and wanted to see if anyone could help break down and understand what this function does step by step?

def label(self, index, *args): 
    """ 
    Label each axes one at a time 
    args are of the form <label 1>,...,<label n> 
    APIPARAM: chxl 
    """ 
    self.data['labels'].append( 
    str('%s:|%s'%(index, '|'.join(map(str,args)) )).replace('None','') 
    ) 
    return self.parent 

Answer 1

It’s a good idea to change the formatting, before trying to understand what it does:

def label(self, index, *args): 
    """ 
    Label each axes one at a time 
    args are of the form <label 1>,...,<label n> 
    APIPARAM: chxl 
    """ 
    self.data['labels'].append( 
        str( '%s:|%s' % \ 
            ( index, '|'.join( map( str,args ) ) ) 
        ).replace( 'None', '' ) 
    ) 
    return self.parent 

So:

it appends something to self.data[ 'labels' ] list. We know this because append() is a method of list object.

This something is a string such that:

• string is of the form xxx:|yyy
• xxx is replaced with the value of argument index
• yyy is replaced with all the other arguments converted to strings (map(str,args)) and joined with | character (join(...)) so resulting in something like ‘a|b|None|c‘
• every occurence of None in the string above is replaced with an empty string and this is appended to the list

EDIT:

As @abarnert pointed out it might be good to explain what does *args mean and why later on it’s used as args, so here it goes.

*args (which is an asterisk + an arbitrary name) means “any number of anonymous arguments available further in args list”. One can also use **kwargs – note two asterisk which is used for accepting keyworded arguments, i.e. ones passed to the function in the form of foo = bar where foo is the name of the argument and bar is its value rather than just bar.

As said above args and kwargs are arbitrary, one could just as well use *potatoes or **potatoes but using args and kwargs is a convention in Python (sometimes people also use **kw instead of **kwargs, but the meaning is the same – any number of anonymous and any number of keyworded arguments respectively).

Both are used if the number of arguments which the function/method should accept is not known beforehand – consider for a example a function which processes names of the party guests, one may not know how many there may be, so defining a following function makes sense:

def add_party_quests( *quests ):
    for guest in quests:
        do_some_processing( guest )

Then both calls below are valid:

add_party_guests( 'John' )
add_party_guests( 'Beth', 'Tim', 'Fred' )

This is also explained in this SO post: https://stackoverflow.com/a/287101/680238