Why Does the Asterisk Flatten Lists in Python\'s `itertools.chain` Function?

How the Asterisk Operator Flattens Lists in Python

In Python, the itertools.chain function can be used to flatten a list of lists. The following code snippet demonstrates this:

<code class="python">uniqueCrossTabs = list(itertools.chain(*uniqueCrossTabs))</code>

However, you may wonder why the asterisk (*) is included in the function call.

Understanding the Asterisk Operator

The asterisk is known as the "splat" operator in Python. It takes an iterable, such as a list, and expands it into actual positional arguments in the function call.

How it Works

Consider the example where uniqueCrossTabs is a list of lists: [[1, 2], [3, 4]]. When you use the asterisk, itertools.chain(*uniqueCrossTabs) expands the list into individual list arguments. This is equivalent to calling itertools.chain([1, 2], [3, 4]).

Comparison to Without the Asterisk

Without the asterisk, you would be passing in just uniqueCrossTabs as a single argument. In this case, chain() would return an iterator that iterates over the list of lists, not the individual elements.

Use of chain.from_iterable()

For flattening lists, itertools.chain.from_iterable() is a more suitable choice. It takes a single iterable of iterables as an argument. Using this method, the code becomes:

<code class="python">uniqueCrossTabs = list(itertools.chain.from_iterable(uniqueCrossTabs))</code>

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