What does function mean?

Function, a general noun indicating a subroutine. In some programming languages, it refers to a subroutine or statement that returns a value. Plays the role of keyword in some programming languages.

In Python, function is very important and common. It generally obtains parameter information of a class or function.

In Python, functions generally fall into the following categories:

1. POSITIONAL_OR_KEYWORD

If there is no * statement , then it is of type POSITIONAL_OR_KEYWORD. Just like the semantics, parameters of type POSITIONAL_OR_KEYWORD can be called by passing parameters in position POSITIONAL, or by passing parameters in keyword KEYWORD. The following is the simplest example:

def foo(a):
pass
# 位置传参调用
foo(1)
# 关键字传参调用
foo(a=1)

2. VAR_POSITIONAL

The second is a variable positional parameter, declared through a * prefix. If you see To a *xxx function parameter declaration (not a function call! Declaration and call have two different meanings), it must be of type VAR_POSITIONAL. Just like semantics, parameters of this type can only be called through position POSITIONAL parameters. The keyword KEYWORD is not supported for parameter passing. Inside the function, the VAR_POSITIONAL type parameters are displayed as a tuple. One thing to note is that the VAR_POSITIONAL type can be called without passing any parameters without reporting an error, and only one is allowed to exist. The following is a simple example:

def foo(*b):
print(b)
# 不传参数不会报错，参数值是一个空元祖
foo() # 结果是 ()
# 可以传入任意个位置参数调用
foo(1, 2.0, '3', True) #结果是 (1, 2.0, '3', True)

3. KEYWORD_ONLY

The third type is keyword parameters. This type of parameter will only be after the VAR_POSITIONAL type parameter and not With ** prefix. Just like semantics, this type of parameters can only be passed using the keyword KEYWORD, not positional parameters, because all positional parameters have been received by the previous VAR_POSITIONAL type parameters, so KEYWORD_ONLY can only receive parameter values ​​through the keyword . The following is a simple example:

# VAR_POSITIONAL不需要使用时，可以匿名化
def foo(*, c):
pass
# 只能关键字传参调用
foo(c=1)

Four. VAR_KEYWORD

The fourth type is variable keyword parameters. Parameters of type VAR_KEYWORD are declared with the ** prefix. (Not a function call! Declaration and call have two different meanings). Like semantics, this type of parameter can only be called through the keyword KEYWORD, but can receive any number of keyword parameters, even 0 parameters, and is displayed as a dictionary (dict) inside the function. Only one parameter of type VAR_KEYWORD is allowed, and it is only allowed to be declared at the end of the function. The following is a simple example:

def foo(**d):
print(d)
# 不传参数不会报错，参数值是一个空字典
foo() # 结果是 {}
# 可以传入任意个关键字参数调用
foo(a=1, b=2.0, c='3', d=True) # 结果是 {'d': True, 'c': '3', 'b': 2.0, 'a': 1}

5. POSITIONAL_ONLY

The fifth type is the positional parameter. I choose this last because it is not important at all and belongs to python. A historical product of With the help of Ipython, you can still see Init signature: dict(self, /, *args, **kwargs) where self is the positional parameter POSITIONAL_ONLY. Trust me, you won't need to use it. Now Python recommends using VAR_POSITIONAL instead. Here is a comprehensive example:

import inspect
def foo(a, *b, c, **d):
pass
for name, parame in inspect.signature(foo).parameters.items():
print(name, ': ', parame.kind)

Default parameters

VAR types do not allow setting default parameters

POSITIONAL_OR_KEYWORD and KEYWORD_ONLY can customize default parameters, while VAR_POSITIONAL and VAR_KEYWORD do not allow customization of default parameters, because the default parameter of VAR_POSITIONAL is tuple() empty tuple, while the default parameter of VAR_KEYWORD is dict() empty dictionary. If you customize the default parameters, you do not need to pass parameters when calling the function. If the default value is empty, you must pass parameters before calling.

The position of the default parameters

The default parameters of the POSITIONAL_OR_KEYWORD type must be placed at the end, otherwise an error will be reported. Although KEYWORD_ONLY is not mandatory, because it is passed using keywords It doesn’t matter who comes first, but it’s best to put it as late as possible.

The default parameters are not default?

Default parameters must not be set to variable types (such as list, dict, set). If you change the default parameters within the function, it will no longer be the default value the next time you call it. .

Correct example:

def foo(p1, p2=2.0, *, k1, k2=None):
a_list = k2 or list()
pass
foo(1, k1='3')

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