Variadic function: Difference between revisions

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Revision as of 18:20, 18 January 2007

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Python supports very flexible variadic functions. By marking variables with one asterisk (e.g. *var) the given variable is defined to be a tuple of all the extra arguments. By marking variables with two asterisks (e.g. **var) the given variable is a dictionary of all extra [[http://en.wikipedia.org/wiki/Parameter_(computer_science) | keyword arguments]]; the keys are strings, which are the names that were . Conventionally these are called "args" and "kwargs" respectively, but they may be something else, and packages often make good use of this ability to improve readability (e.g. [[BeautifulSoup | http://www.crummy.com/software/BeautifulSoup/documentation.html]]). If they exist, these arguments must be the last one in the list.

Python supports very flexible variadic functions. The param

def f(*args, **kwargs):

print args

#args is a [[tuple]] of all the unspecified trailing arguments

print kwargs

#kwargs is a [[dict]] of all the arguments specified using name=val syntax

f(1,2,"cow","kitty")

> (1, 2, "cow", kitty)

> {}

f(arg1=1,sample=2,name="cow",hero="kitty")

> ()

> {"arg1":1, "sample":2, "name":"cow", "hero":"kitty"}

f(1,2,name="cow",hero="kitty")

e.g.

> (1, 2)

> {"name":"cow", "hero":"kitty"}

f(arg1=1,sample=2,name="cow", "kitty") -> SyntaxError "Non-keyword arg after keyword arg"

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Conversely you may also pass in a tuple or dictionary using the same asterisk-notation and have it automatically expand to fill.

def g(a, b, c):

print a, b, c

mytuple = 1,2,3

mydict = {"a":"first", "b":"second", "c":"third"}

g(*mytuple) -> 1 2 3

g(**mydict) -> first second third

g(**{"a":"first"}) -> TypeError "g() takes exactly 3 non-keyword arguments (got 1)"

g(**{"a":"first", "b":"second", "c":"third", "d":"fourth"}) -> TypeError "g() got an unexpected keyword argument 'd'"

==See also==