Python: defining a function with variable number of arguments

Question

I am not sure if this thing has a name, so I couldn't find any information online so far, although surely there is!

Imagine my MWE:

def PlotElementsDict(dictionary1, dictionary2, itemToPlot, title):
    # dictionary 1 and dictionary 2 are collections.OrderedDict with 'key':[1,2,3]
    # i.e. there values of the keys are lists of numbers
    list1 = [dictionary1[key][itemToPlot] for key in dictionary1.keys()]
    list2 = [dictoinary2[key][itemToPlot] for key in dictionary2.keys()]
    plt.plot(list1, label='l1, {}'.format(itemToPlot)
    plt.plot(list2, label = 'l2, {}'.format(itemToPLot')
    plt.legend()
    plt.title(title)
    return plt.show()

How can I create a function (but my question is even more general, I would like to be able to do this for a class as well) which takes a variable number of parameters of a certain type (for example n dictionaries) plus other parameters which you need only one? (for instance item to plot or could be title)?

In practice I would like to create a function (in my MWE) that no matter how many dictionaries I feed into the function, it manages to plot the given items of that dictionary given a common title and item to plot

Possible duplicate of Can a variable number of arguments be passed to a function? — Błotosmętek
– Błotosmętek, Commented Jul 19, 2017 at 13:36
This post also have a good explanation stackoverflow.com/questions/1769403/… — Hamuel
– Hamuel, Commented Jul 19, 2017 at 13:36
You can also use optional/named parameters/arguments, and check to see if the params have the default values after entry to the function, or if the user has entered in other values. diveintopython.net/power_of_introspection/… — silassales
– silassales, Commented Jul 19, 2017 at 13:42

Jonas Adler · Accepted Answer · 2017-07-19 14:13:42Z

2

Solution with asterisk (*)

This would be a perfect case for pythons asterisk argument style, like so:

def PlotElementsDict(itemToPlot, title, *dictionaries):
    for i, dct in enumerate(dictionaries):
        lst = [dct[key][itemToPlot] for key in dct]
        plt.plot(lst, label='l{}, {}'.format(i, itemToPlot))

    plt.legend()
    plt.title(title)
    plt.show()

Example use case:

dct1 = {'key' : [1,2,3]}
dct2 = {'key' : [1,2,3]}
dct3 = {'key' : [1,2,3]}

title = 'title'

itemToPlot = 2

PlotElementsDict(itemToPlot, title, dct1, dct2, dct3)

Arguments in front

If you want the dictionaries to come first, the other arguments have to be keyword only:

def PlotElementsDict(*dictionaries, itemToPlot, title):
    pass

And call it with explicit argument names

PlotElementsDict(dct1, dct2, dct3, itemToPlot=itemToPlot, title=title)

edited Jul 19, 2017 at 14:13

answered Jul 19, 2017 at 13:48

Jonas Adler

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3 Comments

Euler_Salter Over a year ago

Amazing! That is exactly what I was looking for!

Euler_Salter Over a year ago

What should I write when actually calling the function, so that Python understands which are the dictionaries and which are not?

Jonas Adler Over a year ago

Added an example use case. Python understand by the position of the arguments.

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