What does the comma mean in Python's unpack?

Question

We can simply use:

crc = struct.unpack('>i', data)

why do people write it like this:

(crc,) = struct.unpack('>i', data)

What does the comma mean?

Answer 1

The first variant returns a single-element tuple:

In [13]: crc = struct.unpack('>i', '0000')

In [14]: crc
Out[14]: (808464432,)

To get to the value, you have to write crc[0].

The second variant unpacks the tuple, enabling you to write crc instead of crc[0]:

In [15]: (crc,) = struct.unpack('>i', '0000')

In [16]: crc
Out[16]: 808464432

Answer 2

the unpack method returns a tuple of values. With the syntax you describe one can directly load the first value of the tuple into the variable crc while the first example has a reference to the whole tuple and you would have to access the first value by writing crc[1] later in the script.

So basically if you only want to use one of the return values you can use this method to directly load it in one variable.

Answer 3

The comma indicates crc is part of a tuple. (Interestingly, it is the comma(s), not the parentheses, that indicate tuples in Python.) (crc,) is a tuple with one element.

crc = struct.unpack('>i', data)

makes crc a tuple.

(crc,) = struct.unpack('>i', data)

assigns crc to the value of the first (and only) element in the tuple.

Answer 4

(crc,) is considered a one-tuple.