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

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

This function can be called in several ways:

Give only the required Parameters: <span class="pre">ask_ok('Do <span class="pre">you <span class="pre">really <span class="pre">want <span class="pre">to <span class="pre">quit?')<br/></span></span></span></span> </span></span> Give an optional parameter: <span class="pre">ask_ok('OK <span class="pre">to <span class="pre">overwrite <span class="pre">the <span class="pre">file?', <span class="pre">2 )<br/></span></span></span></span></span></span>Or give all parameters: <span class="pre">ask_ok('OK <span class="pre">to <span class="pre">overwrite <span class="pre">the <span class="pre">file?', <span class="pre">2, <span class="pre">'Come <span class="pre">on, <span class="pre">only <span class="pre">yes <span class="pre">or <span class="pre">no!')</span></span></span></span></span></span></span></span></span></span></span></span>

#This example also introduces

in<span class="pre"></span> 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()

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))

This will print

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

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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