The use of python-function function (II.)

Defining functions with optional parameters

Optional parameters can be assigned (by defining = ), the parameter name of the default value:

def make (action='nothing'):    return action

This function can be called in 3 different ways:

Make ('fun ')#  out:funmake(Action="  Sleep")#  out:sleep## # not Passed in. Make ()    # out:nothing

Warning

variable type ( list , dict , set and so on), should be careful when given to be considered default property. Any mutation of the default parameter will change permanently.

LAMBDA (inline/anonymous) functions

the lambda keyword to create an inline function that contains an expression.

Consider features:

def greeting ():     return " Hello "

Wherein, when called:

Print (Greeting ())

Print:

Hello

lambda S can also accept parameters:

Lambda S:s.strip (). Upper () strip_and_upper_case ("  Hello   ")

Return string:

HELLO

They can even take any number of parameters/keyword parameters, such as normal functions.

Greeting = lambda x, *args, **kwargs:print (x, args, Kwargs) greeting (' Hello ', ' world ', world= ' world ')

Print:

Hello ('World',) {'World ' }

The most common use cases lambda are short functions that are used only in one place. For example, this line arranges a list of strings, ignores their case, and ignores spaces at the beginning and end:

They are most commonly used sorted , filter andmap

Sorted (["" "    BAR""BaZ    "  "qux"], key=Lambda s:s.strip (). Upper ())

Return:

['    BAR'BaZ     ' ' Qux ']

==================================================== =========

Sorted (["" "    BAR""BaZ    "  "qux"], key=Lambda s:s.strip ())

Return:

 [ "  BAR   ' ,  '  baz   ' ,  '   foo   ' ,  '  qux   ' ] #   No effect of lambda, list is sorted 

Sorted (["" "    BAR""BaZ    "  "qux"])

Return:

 [ "  BAR   ' ,  '  baz   ' ,  '   foo   ' ,  '  qux   ' ] #   list sorted.  

Sorted (Map (Lambda s:s.strip (). Upper (), ["" "    BAR"  "BaZ    " "qux"]))

Return:

['BAR'BAZ'FOO' Qux ' # What expected

is the first idiom wrong? Can you explain this, please?

==================================================== =====================

My_list = [3, -4,-2, 5, 1, 7]sorted (my_list, key=Lambda x:abs (x))

Return:

[1,-2, 3,-4, 5, 7]

List (filter (lambda x:x>0, my_list)

Return:

[3, 5, 1, 7]

List (map (lambda x:abs (x), my_list)

Return:

[3, 4, 2, 5, 1, 7]

Other functions (with/without parameters) can be called from within a lambda function.

def foo (msg):     Print  Lambda"helloWorld": foo (x) greet ()

Print:

Hello World

Because this is very useful lambda can only contain one expression and can run multiple statements by using one of the subordinate functions.

Attention

Remember, PEP-8 ( The official Python style guide) does not recommend that a lambda expression be assigned to a variable (as in the first two examples):

Always use the DEF statement instead of binding the lambda expression directly to the assignment statement of the identifier.

Is:

def return 2*x

No:

Lambda x:2*x

The first form refers to the resulting function where the name of the object is special f , rather than the generic <lambda> . This is more useful for general tracking and string representations. The only good thing to eliminate with an assignment statement is that a lambda expression can provide an explicit def declaration (that is, it can be embedded in a larger expression).

The use of python-function function (II.)