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First class objects and first class functions

stay Python in , Integers 、 Strings and dictionaries are first-class objects . Functions are also first-class objects . People often will “ Treat functions as first-class objects ” Referred to as “ The wait function ”.

Programming language theorists “ First class object ” Defined as a procedural entity that meets the following conditions ：

• Create at run time
• Can be assigned to an element in a variable or data structure
• Can be passed to a function as a parameter
• Can be the return result of a function

Higher order function

Accept a function as an argument , Or the function returned as a result is a higher-order function . Such as map and sorted .

In the functional programming paradigm , The most well-known higher-order functions are map 、filter 、reduce and apply .

stay Python 3 in ,map and filter Return to generator （ An iterator ）. List derivation or generator expression has map and filter The function of two functions , And it's easier to read . Take a chestnut ：

list(map(fact, range(6)))		# [1, 1, 2, 6, 24, 120]  Factorial 
[fact(n) for n in range(6)]		# [1, 1, 2, 6, 24, 120]

list(map(factorial, filter(lambda n: n % 2, range(6)))) 	# [1, 6, 120]
[factorial(n) for n in range(6) if n % 2] 					# [1, 6, 120]
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stay Python 2 in ,reduce It's a built-in function , But in Python 3 Put in functools In the module .

apply Function in Python 2.3 Marked obsolete in , stay Python 3 Removed from , Because I don't need it anymore . If you want to call a function with non quantitative parameters , You can write fn(*args, **keywords) , No more writing apply(fn, args, kwargs) .

Anonymous functions

lambda The key word in Python Create anonymous function within expression .lambda Syntax is just grammar sugar ： And def The sentence is the same ,lambda Expressions create function objects .

Properties of functions

Use dir The function can view the properties of the function object ：dir(function_name)

An instance of a class has no properties that a function has ：