Learning Notebook - David Rostcheck
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A single star * unpacks a sequence or collection into positional arguments. Suppose we have def add(a, b): return a + b values = (1, 2) Using the * unpacking operator, we can write s = add(*values), which will be equivalent to writing s = add(1, 2). The double star ** does the same thing for a dictionary, providing values for named arguments: values = { 'a': 1, 'b': 2 } s = add(**values) # equivalent to add(a=1, b=2) Both operators can be used for the same function call. For example, given: def sum(a, b, c, d): return a + b + c + d values1 = (1, 2) values2 = { 'c': 10, 'd': 15 } then s = add(*values1, **values2) is equivalent to s = sum(1, 2, c=10, d=15). See also the relevant section of the tutorial in the Python documentation. Similarly, * and ** can be used for parameters. Using * allows a function to accept any number of positional arguments, which will be collected into a single parameter: def add(*values): s = 0 for v in values: s = s + v return s Now when the function is called like s = add(1, 2, 3, 4, 5), values will be the tuple (1, 2, 3, 4, 5) (which, of course, produces the result 15). Similarly, a parameter marked with ** will receive a dict: def get_a(**values): return values['a'] s = get_a(a=1, b=2) # returns 1 this allows for specifying a large number of optional parameters without having to declare them. Again, both can be combined: def add(*values, **options): s = 0 for i in values: s = s + i if "neg" in options: if options["neg"]: s = -s return s s = add(1, 2, 3, 4, 5) # returns 15 s = add(1, 2, 3, 4, 5, neg=True) # returns -15 s = add(1, 2, 3, 4, 5, neg=False) # returns 15
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