What Does the Comma Do in Python Assignments Like \'x, = ...\'?

Is "x, = ..." in Python the Comma Operator?

In Python, a comma after variable lines in an assignment statement may evoke questions about its purpose. Contrary to intuition, it is not the comma operator that merges multiple expressions or separators, but rather an indicator of unpacking a single-element tuple returned by the function call.

For instance, in the code sample:

<code class="python">line, = ax.plot(x, np.sin(x))</code>

ax.plot() returns a tuple containing a single Line2D object. The comma in this line unpacks the tuple and assigns the element to the variable line.

This pattern is commonly used when dealing with functions that return multiple values. Consider the following example:

<code class="python">base, ext = os.path.splitext(filename)</code>

Here, os.path.splitext() returns a tuple with two elements, assigned to the variables base and ext.

The comma in the assignment target can be omitted when the target is a list or tuple that matches the number of elements in the unpacked tuple:

<code class="python">(line,) = ax.plot(x, np.sin(x))</code>

Alternatively, you can avoid unpacking altogether by accessing the first element of the returned tuple directly:

<code class="python">line = ax.plot(x, np.sin(x))[0]</code>

The use of tuple unpacking in assignments offers a concise and readable way to assign multiple variables at once, especially when dealing with functions that return multiple values.

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