Введение в Python

Теория: Python functions

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Functions in Python

Common structure

def name(arg1, ... argN, kwarg1, ... kwargN, *args, **kwargs):
    """Short description
       Long description...
    """
....operator1
....operatorN
....return value

Naming rules

lower_underscore_case
letters, numbers, underscores
leading underscore for private funcs

myFunc()          # bad
My_func           # bad
100_top_users     # SyntaxError
get_top_users()   # ok
_fetch_data()     # perhaps you shouldn't call that directly

Why need docstring

Keep code and docs at the same place
Useful for IDEs, editors
Build documentation from code
help(func) or func.doc prints the doc
Doctests

def test(a, b, c=3, d=4)

test()                   TypeError
test(1, 2)               a=1, b=2, c=3, d=4
test(4, 10, c=22)        a=4, b=10, c=22, d=4
test(a=6, b=5)           a=6, b=5, c=3, d=4
test(d=4, a=1)           TypeError
test(c=1, b=4, a=5)      a=5, b=4, c=1, d=4
test(9, 8, 7)            a=9, b=8, c=7, d=4
test(9, 8, 7, 6)         a=9, b=8, c=7, d=6

What does function return

- Use return <value> statement
- There might be several return statements
- None if there is no return statement
- Use tuple to return multiple values

def func(foo, bar, baz=42):
    if baz == 42:
        return (False, "some-error")
    else:
        return (True, "OK")

ok, message = func(1, 2)            False, "some-error"
ok, message = func(1, 2, baz=3)     True, "OK"

Function is object

- Copy, delete function
- Pass it into a function call
- Might be either a key or value of a dict
- Has special attrs (__name__, __module__, etc)

def user_auth(user):
    pass

def user_logout(user)
    pass

actions = {"auth": user_auth, "logout": user_logout}

actions[action](user)

args, kwargs

def (*args, **kwargs)
aggregate rest arguments and keyword arguments
args is a tuple, kwargs is a dict
useful when you construct arguments step by step

params = {"name": "Ivan", "age": 29}
if ...
    params["created_at"] = datetime.now()
if ...
    params["friends"] = ["Joe", "Mary"]

create_user(**params)
create_user(name="Ivan", age=29,
                    created_at=datetime.now(),
                    friends=["Joe", "Mary"]
)

Never use mutable defaults!

def test(a=[]):
    a.append(1)
    print a

test()     [1]
test()     [1, 1]
test()     [1, 1, 1]
test()     [1, 1, 1, 1]

Lambdas

Came from functional programming
A short expression with only one statement inside
Pros:

just-in-place function
supports argument unpacking

Cons:

long ones are ugly
hard to debug

double = lambda x: 2 * x
map(double, [1, 2, 3])                  [2, 4, 6]

calc = lambda (x, y, z): x * y - z
map(calc, [(1, 2, 3), (4, 5, 6)])       [-1, 14]

avg = lambda *args: sum(args) / float(len(args))
avg(1, 2, 3)                            2.0

lambda x: pass                          SyntaxError

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