Why Must Default Arguments Come Before Non-Default Arguments in Python Functions?

Why Must Default Arguments Precede Non-Default Arguments?

In Python, the order of arguments in function definitions is crucial, especially when a mix of default and non-default arguments is involved. This ordering rule ensures the unambiguous interpretation of arguments during function calls.

When a non-default argument follows a default argument in the function's definition, as in the following example, a SyntaxError is thrown:

<code class="python">def fun1(a="who is you", b="True", x, y):
    print(a, b, x, y)</code>

This error occurs because required parameters (non-default) must always precede optional parameters (default) in the function signature. Default arguments fulfill placeholder values when arguments are omitted in function calls, while non-default arguments are mandatory.

If arguments are placed in the incorrect order, as seen in the above example, the interpreter cannot determine the correct assignment of values to function parameters. It is not clear whether the value passed during a function call would match the default or non-default argument.

To avoid ambiguity, the following correct syntax can be used:

<code class="python">def fun1(x, y, a="who is you", b="True"):
    print(a, b, x, y)</code>

Keyword arguments can be used along with default arguments to provide out-of-order positional arguments or skip missing arguments. For example, the default arguments "a" and "b" can be used with the following function call:

<code class="python">fun1(1, 2, b="False")</code>

In this case, the first two positional arguments are assigned to "x" and "y" respectively, while the keyword argument assigns the value "False" to "b", skipping the default value "True".

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