Function objects in the python advanced tutorial (functions are also objects)

This article mainly introduces the function objects in the python advanced tutorial. Function objects refer to functions as well as objects. This article also describes lambda functions, functions as parameter passing, map () functions, filter () functions, reduce () functions and other content. If you need them, you can refer to the idea that everything is an object. Let's look back at the function ). A function is also an object with attributes (which can be queried using dir ). As an object, it can also be assigned to other object names or passed as a parameter.

Lambda Functions

Before expanding, we should first mention the lambda function. You can use the syntax of the lambda function to define the function. Lambda example:

The Code is as follows:

Func = lambda x, y: x + y
Print func (3, 4)

Lambda generates a function object. The return value is x + y. Function objects are assigned to func. The call of func is no different from that of a normal function.

The preceding definition can be written as follows:

The Code is as follows:

Def func (x, y ):
Return x + y

Passing functions as parameters

A function can be used as an object to transmit parameters. Function Name (such as func) is the object. For example:

The Code is as follows:

Def test (f, a, B ):
Print 'test'
Print f (a, B)

Test (func, 3, 5)

The first parameter f of the test function is a function object. Pass func to f. f () in test has the func () function.

Therefore, we can improve program flexibility. You can use the test function above to bring different function parameters. For example:

The Code is as follows:

Test (lambda x, y: x ** 2 + y), 6, 9)

Map () function

Map () is a built-in function of Python. Its first parameter is a function object.

The Code is as follows:

Re = map (lambda x: x + 3), [1, 3, 5, 6])

Here, map () has two parameters: a function object defined by lambda and a table containing multiple elements. Map () is used to act on each element of a table in sequence, and the results of each function are stored in the returned table re. Map uses the READ function (lambda function) to operate data (here "data" is each element in the table, and "operation" is to add 3 to each data ).

In Python 3. X, the returned value of map () is a circular object. You can use the list () function to convert the circular object into a table.

If the function object as a parameter has multiple parameters, you can use the following method to pass multiple parameters of the function parameter to map:

The Code is as follows:

Re = map (lambda x, y: x + y), [1, 2, 3], [6, 7, 9])

Map () extracts one element from two tables each time and brings it into the function defined by lambda.

Filter () function

The first parameter of the filter function is also a function object. It also acts on multiple elements as function objects of parameters. If the return value of the function object is True, the element is stored in the returned table. Filter filters data by reading functions. Similarly, in Python 3. X, filters return loop objects instead of tables.

The filter function is used as follows:

The Code is as follows:

Def func ():
If a> 100:
Return True
Else:
Return False

Print filter (func, [101,500,])

Reduce () function

The first parameter of the reduce function is also a function, but there is a requirement that the function itself can receive two parameters. Reduce can repeatedly act on each parameter. For example:

The Code is as follows:

Print reduce (lambda x, y: x + y), [1, 2, 5, 7, 9])

The first parameter of reduce is the lambda function, which receives two parameters x, y and returns x + y.

Reduce transfers the first two elements (1 and 2) of the table to the lambda function. 3 is displayed. The returned value (3) is used as the first parameter of the lambda function, and the next element (5) in the table is used as the second parameter of the lambda function, get 8. Call the lambda function in sequence. The first parameter of each lambda function is the result of the previous operation, and the second parameter is the next element in the table until there is no remaining element in the table.

The above example is equivalent to (1 + 2) + 5) + 7) + 9

According to mmufhy's reminder, the reduce () function cannot be used directly in 3.0. It is defined in the functools package and needs to be introduced. See the comment area.

Summary

A function is an object.

Define functions using lambda

Map ()

Filter ()

Reduce ()