Why Do I Get a Syntax Error When Defining Nested Arguments in Python 3?

Python 3: Syntax Error While Defining Nested Arguments

Error Context

Developers may encounter a syntax error when compiling Python code into a module due to the attempted definition of nested arguments. While the same code runs seamlessly in IDLE using the "Run module" option, an error similar to the following may arise during distribution:

SyntaxError: invalid syntax
File "/usr/local/lib/python3.2/dist-packages/simpletriple.py", line 9
def add(self, (sub, pred, obj)):
                  ^

Cause

This error occurs because of the removal of tuple argument unpacking in Python 3 as explained in PEP 3113.

Solution

To rectify this error, the code should be modified to pass the tuple as a single parameter and unpack it manually. The affected code, def add(self, (sub, pred, obj)):, should be revised as follows:

def add(self, sub_pred_obj):
    sub, pred, obj = sub_pred_obj

For lambda functions, it is generally preferable to avoid unpacking altogether. Instead of using:

lambda (x, y): (y, x)

It is recommended to write:

lambda xy: (xy[1], xy[0])

Automated Tools

To facilitate the detection and correction of this issue, developers can utilize programs such as "2to3," "modernize," or "futurize" to refactor their Python 2.x code to Python 3.x, effectively identifying and suggesting suitable solutions for nested argument handling.

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