是否可以将默认参数传递给 for 循环？

Question

I have a __str__ method that takes some optional parameters (default is set to None). If these parameters are not specified, it returns a str that shows the full matrix. If the optional parameters are specified, it returns a partial matrix corresponding to those values.

def __str__(self, starting_row=None, starting_col=None, nrows=3, ncols=3):
    # remove that final space
    string = ""
    for i in range(starting_row, nrows):
        for j in range(starting_col, ncols):
            string += str(self.get(i, j))
        string += "\n"
    return string

My code works if starting_row and starting_col are specified to integer values (eg starting_row=1 ). However, if these values are not specified, since the default value is None, I am getting an error.

What would be the most pythonic way to specify a starting_row and starting_column of 0 ONLY IF the user does not specify the starting_row within the function parameters? Is there a way to do so in the for loop parameters?

Answer 1

Just change the default values for the parameters.
This is because in your case, the default value None for starting_row has no meaning, according to you. Since you want it to default to 0, then do just that.

def __str__(self, starting_row=0, starting_col=0, nrows=3, ncols=3):
    string = ""
    for i in range(starting_row, nrows):
        for j in range(starting_col, ncols):
            string += str(self.get(i, j))
        string += "\n"
    return string

Pedantic note: String concatenation like that is expensive, because strings aren't mutable in python so you're essentially creating new strings in every step. A better alternative would be to append those values to a list and then perform a join at the end.

Pedantic note2: Also, this most likely is a method belonging to your class(as you have a self in there), so perhaps a cleaner way to go about it would be to not have parameters at all for the __str__() and instead make use of instance attributes