python args operator and map function

Question

How does args work? I know it is used when you don't know how many arguments you may pass to a function, but I need deeper explanation. Do they run function for each argument passed?

    def buy_sell_hold(*args):
        cols = [c for c in args]
        requirement = 0.02
        for col in cols:
            if col > requirement:
                return 1
            if col < -requirement:
                return -1
        return 0
    def extract_featuresets(ticker):
        tickers, df = process_data_for_labels(ticker)

        df['{}_target'.format(ticker)] = list(map( buy_sell_hold,
                                               df['{}_1d'.format(ticker)],
                                               df['{}_2d'.format(ticker)],
                                               df['{}_3d'.format(ticker)],
                                               df['{}_4d'.format(ticker)],
                                               df['{}_5d'.format(ticker)],
                                               df['{}_6d'.format(ticker)],
                                               df['{}_7d'.format(ticker)] ))
    vals = df['{}_target'.format(ticker)].values.tolist()
    return vals

Vals are single list how so, I'm lost.

These questions have been asked multiple times, it is advisable to search the already available knowledge base of SO, or Google for that matter, before asking a question. See [How much research effort is expected of Stack Overflow users?](https://meta.stackoverflow.com/q/261592/5431791) e.g. answer on [`map`](https://stackoverflow.com/q/10973766/5431791) — Sayandip Dutta, Jan 17 '21 at 18:44

score 0 · Answer 1 · edited Jan 17 '21 at 18:45

python has two types of variable length arguments:

*args (Non Keyword Arguments)
**kwargs (Keyword Arguments)

When you define a function with *args, you effectively take arguments and collect them into a list. So;

def foo(*a):
    # a here is a tuple: (2,3,4,5)
    print(a)

foo(2,3,4,5)
>>>
(2,3,4,5)

When you define a function with **kwargs, you effectively take named arguments and collect them into a dict. So;

def zoo(**b):
    # b here is a dict: {'test': 2, 'abc': 'def'}
    print(b)
zoo(test=2, abc='def')
>>>
{'test': 2, 'abc': 'def'}

When you want to pass variable arguments from a list or a dict, you can use similarly:

def xxx(a=2, b=4):
    print(a+b)
kw = {'a': 5, 'b': 7}
xxx(**kw)
>>>
12

see: https://www.programiz.com/python-programming/args-and-kwargs#:~:text=Python%20has%20*args%20which%20allow,to%20pass%20variable%20length%20arguments.

python args operator and map function

1 Answers1