Value that was calculated in the beginning of a function isn't remembered later on in the same function - function

Question

In the beginning of the function I calculate the total weight of a protein sequence and define it as seq_weight.
After that I calculate the weight of several fragments and make combinations of those weights that sum to the total weight of the first proteins sequence.
The first print statement prints the total weight correctly, but near the end of the function it seems to forget that value when I want to define it as the result of the sum.
When I type the value manually I get the result I want:
def fragmentcombinations(sequence, fragments):
for seq_rec in sequence:
seq_weight = 0.0
for i in seq_rec.seq:
seq_weight += SeqUtils.molecular_weight(i, "protein")
print("The protein sequence: " + seq_rec.seq)
print("The molecular weight: " + str(round(seq_weight, 2)) + " Da.")
nums = []
for a in fragments:
fragment_weights = 0.0
for aa in a.seq:
fragment_weights += SeqUtils.molecular_weight(aa, 'protein')
nums.append(round(fragment_weights, 2))
print(nums)
weights_array = []
combs = []
if len(nums) > 0:
for r in range(0,len(nums)+1):
weights_array += list(combinations(nums, r))
for item in weights_array:
if sum(item) == 4364.85: #Sequence weight needs to inserted manually -> not ideal
combs.append(item)
print(" ")
print("The possible combinations of fragment weights that may cover the protein sequence without overlap are: ")
for row in combs:
print(*row, sep = ", ")
fragmentcombinations(seq_list3, seq_list4)
This is the result:
The protein sequence: IEEATHMTPCYELHGLRWVQIQDYAINVMQCL
The molecular weight: 4364.85 Da.
[3611.86, 2269.63, 469.53, 556.56, 1198.41, 2609.88, 547.69, 1976.23, 2306.48, 938.01, 1613.87, 789.87, 737.75, 2498.71, 2064.25, 1184.39, 1671.87]
The possible combinations of fragment weights that may cover the protein sequence without overlap are:
556.56, 1198.41, 2609.88
469.53, 2609.88, 547.69, 737.75
556.56, 1198.41, 938.01, 1671.87
469.53, 547.69, 938.01, 737.75, 1671.87
If I write
if sum(item) == seq_weight:
the result doesn't print the combination of weights like I intended.
Sorry if the code is kind of messy, I'm still a beginner.
Thanks in advance!

Answer 1

The problem is not that your variable is not remembered anymore. The problem is that you perform an exact comparison between floating point numbers. In programming floating point numbers are the "decimal" numbers, but they are not the exact presentation of your numbers. They only are up to an arbitrary precision.
Let's do some basic maths Python.
>>> a = 0.2 + 0.1
>>> a
0.30000000000000004
>>> a == 0.3
False
As you can see, there is clearly happening something weird here. But this is just how floating point arithmetic works.
Now we have explained that. What should you do to make your program work? There are multiple solutions.
One way to deal with it, is to compare your numbers to some fixed difference. ie
if abs(sum(item) - seq_weight) < 0.00001
Another way to deal with this is using fixed precision decimal objects, but that can be more difficult than you think it is. https://docs.python.org/3/library/decimal.html