what does the comma mean in python's unpack?

问题

we can simply use:

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

why people like this:

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

what does the comma mean?

回答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

回答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.

回答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.