The *args
and **kwargs
is a common idiom to allow arbitrary number of arguments to functions as described in the section more on defining functions in the Python documentation.
The *args
will give you all function parameters as a tuple:
def foo(*args):
for a in args:
print(a)
foo(1)
# 1
foo(1,2,3)
# 1
# 2
# 3
The **kwargs
will give you all
keyword arguments except for those corresponding to a formal parameter as a dictionary.
def bar(**kwargs):
for a in kwargs:
print(a, kwargs[a])
bar(name='one', age=27)
# name one
# age 27
Both idioms can be mixed with normal arguments to allow a set of fixed and some variable arguments:
def foo(kind, *args, **kwargs):
pass
It is also possible to use this the other way around:
def foo(a, b, c):
print(a, b, c)
obj = {'b':10, 'c':'lee'}
foo(100,**obj)
# 100 10 lee
Another usage of the *l
idiom is to unpack argument lists when calling a function.
def foo(bar, lee):
print(bar, lee)
l = [1,2]
foo(*l)
# 1 2
In Python 3 it is possible to use *l
on the left side of an assignment (Extended Iterable Unpacking), though it gives a list instead of a tuple in this context:
first, *rest = [1,2,3,4]
first, *l, last = [1,2,3,4]
Also Python 3 adds new semantic (refer PEP 3102):
def func(arg1, arg2, arg3, *, kwarg1, kwarg2):
pass
Such function accepts only 3 positional arguments, and everything after *
can only be passed as keyword arguments.
Note:
- A Python
dict
, semantically used for keyword argument passing, are arbitrarily ordered. However, in Python 3.6, keyword arguments are guaranteed to remember insertion order.
- "The order of elements in
**kwargs
now corresponds to the order in which keyword arguments were passed to the function." - What’s New In Python 3.6
- In fact, all dicts in CPython 3.6 will remember insertion order as an implementation detail, this becomes standard in Python 3.7.
In Python 3.x, 5 / 2
will return 2.5
and 5 // 2
will return 2
. The former is floating point division, and the latter is floor division, sometimes also called integer division.
In Python 2.2 or later in the 2.x line, there is no difference for integers unless you perform a from __future__ import division
, which causes Python 2.x to adopt the 3.x behavior.
Regardless of the future import, 5.0 // 2
will return 2.0
since that's the floor division result of the operation.
You can find a detailed description at https://docs.python.org/whatsnew/2.2.html#pep-238-changing-the-division-operator
Best Answer
($!)
is strict function application. That is, it evaluates the argument before evaluating the function.This is contrary to normal lazy function application in Haskell, e.g.
f x
orf $ x
, which first start to evaluate the functionf
, and only compute the argumentx
if it is needed.For example
succ (1 + 2)
will delay the addition1 + 2
by creating a thunk, and start to evaluatesucc
first. Only if the argument to succ is needed, will1 + 2
be evaluated.However, if you know for sure that the argument to a function will always be needed, you can use
($!)
, which will first evaluate the argument to weak head normal form, and then enter the function. This way, you don't create a whole big pile of thunks and this can be more efficient. In this example,succ $! 1 + 2
would first compute3
and then enter the functionsucc
.Note that it is not always safe to just replace normal function application with strict function application. For example: