学习递归，错误消息(默认参数后接非默认参数)python [英] Learning recursion, error message(non-default argument follows default argument) python

问题描述

尝试练习学习递归.该程序旨在遍历列表并打印出所选字母的位置.我也不明白此错误消息的含义:(非默认参数紧随默认参数之后).不太了解为什么有错误或如何解决它.任何帮助总是感激不尽，谢谢.

Trying to practice learning recursion. The program is designed to go through the list and print out the location of the chosen letter. I also don't understand what this error message means:(non-default argument follows default argument). Not really understanding why there is an error or how to fix it. Any help always appreciated, thanks.

def get_position(pos = 0,List,letter = 'o'):

    if pos < 0:
        pos = 0
    if pos == len(List):
        return None

    if List[pos] is letter:
        return pos
    else:
        return get_position(pos +1,List,letter)


print('position: ',get_position(0,'hello','o'))    

推荐答案

问题出在此行:

get_position(pos = 0,List,letter = 'o')

请注意，您在默认参数pos = 0之后提供了List参数，这是一个非默认参数.这是不允许的.尝试重新排列参数:

Notice that you give the List argument, a non-default argument, after default argument pos = 0. This is not allowed. Try reordering your arguments:

get_position(List, pos = 0,letter = 'o')

要理解为什么不允许这样做，请考虑以下问题:如何调用get_position并传递 second 参数而不传递 first 参数?您可以想到某种语言可以支持这一点的方式，例如允许像get_position( , myList)这样的调用，但是据我所知，没有语言可以支持.

To understand why this is not allowed, think about this: how would you call get_position and pass in the second argument without passing in the first argument? You can think of some ways a language could support this, such as allowing calls like get_position( , myList), but to my knowledge there is no language that does.

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