Python - Assigning 2 variables from one string

Question

values is an array; eventTokens is a string (first element of values). What does the double assignment do? (What are the values of eventToken1 & eventToken2?)

values = data.split("\x01")

eventTokens = values.pop(0)
eventToken1, eventToken2 = eventTokens

I've done an output task (on the Python source) that resulted in the following:

eventTokens is →☹
eventToken1 is →
eventToken2 is ☹

I concluded that the vars somehow split the initial string. However, if I tried compiling an (apparently) similar thing:

arr = ["some", "elements", "inarray"]
c = arr.pop(0)
a, b = c
print c
print a
print b

It resulted in an exception: ValueError: too many values to unpack .

Note: print is not a parameterized method in the tested environment

Answer 1

Variable unpacking is the Python's ability of multiple variable assignment in a single line. The constraint is that the iterable on right side of the expression have to be the same lenght of the variables on the left side. Otherwise you get a too many or to little values to unpack exception.

If you have a string of size 2 like eventTokens is supposed to be, you can then:

>>>a,b = 'ab'
>>>a
'a'
>>>b
'b'

This is very unsafe code. If somehow eventTokens grows larger than two elements the code will raise an exception an your program will be shut down.

Hope this helps!

Answer 2

Since eventTokens is a string of length two, it can be unpacked into two single character strings:

>>> a, b = 'ab'
>>> a
'a'
>>> b
'b'

However, the number of characters in the string must match the number of variables being unpacked into:

>>> a, b = 'abcd'
ValueError: too many values to unpack

---

Note that you can unpack into one variable!

>>> a, = 'x'
>>> a
'x'
>>> a, = 'xyz'
ValueError: too many values to unpack