Why Do Non-Default Arguments Have to Precede Default Arguments in Python Function Definitions?

Unveiling the Syntax Error in Function Argument Ordering

Why does the following Python code encounter a SyntaxError?

def fun1(a="who is you", b="True", x, y):
    print a,b,x,y

The reason for the error is that non-default arguments (x and y) must precede default arguments (a and b) in the function signature. Required parameters must appear before optional ones with default values.

The Danger of Mixed Modes

Allowing non-default arguments to follow default arguments would introduce ambiguity for the interpreter in determining which values correspond to which arguments. Consider the following scenario:

def fun1(a, b, x="who is you", y="True"):
    ...

Suppose we call this function with the following arguments:

fun1(1, "ok")  # 1 assigned to x, "ok" assigned to a or b?

Without the ordering rule, it would be impossible to assign values to arguments unambiguously.

Correct Syntax for Function Arguments

To define functions correctly, default arguments must always follow non-default arguments. The correct syntax for the example function is:

def fun1(x, y, a="who is you", b="True"):
    ...

This syntax ensures that the interpreter can assign values to arguments in a deterministic manner, even in the presence of keyword arguments and missing arguments.

Keyword Arguments Rescue

Keyword arguments allow you to override default values by specifying the argument name explicitly. This is particularly useful for skipping missing arguments. For example, consider the following call to fun1:

fun1(x=1, b="False")  # Override default value of b

Conclusion

The restriction on non-default arguments following default arguments ensures clear and unambiguous interpretation of function calls, preventing syntax errors and runtime errors due to incorrect argument assignment.

The above is the detailed content of Why Do Non-Default Arguments Have to Precede Default Arguments in Python Function Definitions?. For more information, please follow other related articles on the PHP Chinese website!