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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.
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