Getting

Question

I have 2 lists:

first_lst = [(\'-2.50\', 0.49, 0.52), (\'-2.00\', 0.52, 0.50)]
second_lst = [(\'-2.50\', \'1.91\', \'2.03\'), (\'-2.00\', \'1.83\', \'2.08\')         


        

Answer 1

You need to use tuple() or list() to convert that generator expression to a list or tuple:

[tuple((fir[0], fir[1]*sec[1], fir[2]*sec[2]) for fir in first_lst)\
                               for sec in second_lst if fir[0] == sec[0]]

Working version of your code:

>>> first_lst = [tuple(float(y) for y in x) for x in first_lst]
>>> second_lst = [tuple(float(y) for y in x) for x in second_lst]

>>> [((fir[0],) + tuple(x*y for x, y in zip(fir[1:], sec[1:]))) \
                  for fir in first_lst for sec in second_lst if fir[0]==sec[0]]
[(-2.5, 0.9359, 1.0555999999999999), (-2.0, 0.9516000000000001, 1.04)]

Answer 2

Considering that your first_lst and second_lst are defined as follows.

>>> first_lst = [('-2.50', '0.49', '0.52'), ('-2.00', '0.52', '0.50')]
>>> second_lst = [('-2.50', '1.91', '2.03'), ('-2.00', '1.83', '2.08')]

The following list comprehension may be useful.

>>> [tuple((float(elem[0][0]), float(elem[0][1])*float(elem[1][1]), float(elem[0][2])*float(elem[1][2]))) for elem in zip(first_lst, second_lst) if elem[0][0]==elem[1][0]]
[(-2.5, 0.9359, 1.0555999999999999), (-2.0, 0.9516000000000001, 1.04)]

Answer 3

There are 2 issues to look at.

The original code will generate the error:

>>> first_lst = [('-2.50', 0.49, 0.52), ('-2.00', 0.52, 0.50)]
>>> second_lst = [('-2.50', '1.91', '2.03'), ('-2.00', '1.83', '2.08')]
>>> [((fir[0], float(fir[1])*float(sec[1]), float(fir[2])*float(sec[2])) for fir in first_lst) for sec in second_lst if fir[0] == sec[0]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
NameError: name 'fir' is not defined
>>>

and <generator object <genexpr> message is mentioned.

1) Let's fix the the code with minimum amount of changes by creating list comprehension:

>>> first_lst = [('-2.50', 0.49, 0.52), ('-2.00', 0.52, 0.50)]
>>> second_lst = [('-2.50', '1.91', '2.03'), ('-2.00', '1.83', '2.08')]
>>> [(fir[0],fir[1]*float(sec[1]),fir[2]*float(sec[2])) for fir in first_lst for sec in second_lst if fir[0] == sec[0]] # list comprehension
[('-2.50', 0.9359, 1.0555999999999999), ('-2.00', 0.9516000000000001, 1.04)]
>>>

2) In the original code, the bracket after first_lst ) is misplaced. If we place that bracket after the sec[0] instead of list comprehension we get generator expression. And that will cause the <generator object <genexpr> message:

>>> [((fir[0],fir[1]*float(sec[1]),fir[2]*float(sec[2])) for fir in first_lst for sec in second_lst if fir[0] == sec[0])]  # generator object
[<generator object <genexpr> at 0x00000184EEDE29E8>]

In terms of syntax, the only difference is that one uses parenthesis instead of square brackets.

Note: If needed, there are two ways to convert a generator object to the list:

2a) Use asterisk (*) operator to unpack object to the list

>>> [*((fir[0],fir[1]*float(sec[1]),fir[2]*float(sec[2])) for fir in first_lst for sec in second_lst if fir[0] == sec[0])]
[('-2.50', 0.9359, 1.0555999999999999), ('-2.00', 0.9516000000000001, 1.04)]
>>>

2b) Use explicitly list()

>>> list((fir[0],fir[1]*float(sec[1]),fir[2]*float(sec[2])) for fir in first_lst for sec in second_lst if fir[0] == sec[0])
[('-2.50', 0.9359, 1.0555999999999999), ('-2.00', 0.9516000000000001, 1.04)]
>>>

Answer 4

i had the same problem and found a simpler answer for your question. only thing you need to do is using original for loop syntax and it works nicely!

this is working version of your code:

ans=[]
for fir in first_lst:
    for sec in second_lst:
        if float(fir[0])==float(sec[0]):
             ans.append([fir[0],float(fir[1])*float(sec[1]),float(fir[2])*float(sec[2])])

print(ans)

output=[['-2.50', 0.9359, 1.0555999999999999], ['-2.00', 0.9516000000000001, 1.04]]