Usage and key points to note about the default values of Python function parameters-Python Tutorial-php.cn

Usage and key points to note about the default values of Python function parameters

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Release： 2019-04-15 11:07:29

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This article brings you the usage and key points to note about the default values of python function parameters. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. help.

The most useful form is to specify a default value for one or more parameters. The function thus created can be called with fewer arguments than allowed when defined, for example:

def ask_ok(prompt, retries=4, reminder=&#39;Please try again!&#39;):
    while True:
        ok = input(prompt)
        if ok in (&#39;y&#39;, &#39;ye&#39;, &#39;yes&#39;):
            return True
        if ok in (&#39;n&#39;, &#39;no&#39;, &#39;nop&#39;, &#39;nope&#39;):
            return False
        retries = retries - 1
        if retries < 0:
            raise ValueError(&#39;invalid user response&#39;)
        print(reminder)

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This function can be called in several ways:

Give only the required Parameters: ask_ok('Do you really want to quit?')  Give an optional parameter: ask_ok('OK to overwrite the file?', 2 ) Or give all parameters: ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')

#This example also introduces

in keyword. It can test whether a sequence contains a certain value.

The default value is calculated at the function definition in the

definition process, so

i = 5

def f(arg=i):
    print(arg)

i = 6
f()

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will print 5.

Important warning: The default value will only be executed once. This rule is important when the default value is a mutable object (Python lists, dictionaries, and most class instances). For example, the following function will store the arguments passed to it on subsequent calls:

def f(a, L=[]):
    L.append(a)
    return L

print(f(1))
print(f(2))
print(f(3))

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This will print

[1]
[1, 2]
[1, 2, 3]

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If you don't want the default value to be shared between subsequent calls, you can Write this python function like this:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

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Usage and key points to note about the default values ​​of Python function parameters

Usage and key points to note about the default values of Python function parameters