writing a generator that yields dictionaries of base frequencies of nucleotides - python-3.x

Question

I am trying to write a function that returns a generator that can be iterated over all starting position of a k-window in the DNA sequence. For each starting position, the generator returns the nucleotide frequencies in the window as a dictionary.
def sliding(s,k):
d = {}
for i in range(len(s)-3):
chunk = ''.join([s[i],s[i+(k-3)],s[i+(k-2)],s[i+(k-1)]])
for j in chunk:
if j not in d:
d[j] = 1
else:
d[j] += 1
yield d
seq = "ACGTTGCA"
for d in sliding(seq,4):
print(d)
Output:
{'A': 1, 'C': 1, 'G': 1, 'T': 1}
{'A': 1, 'C': 2, 'G': 2, 'T': 3}
{'A': 1, 'C': 2, 'G': 4, 'T': 5}
{'A': 1, 'C': 3, 'G': 5, 'T': 7}
{'A': 2, 'C': 4, 'G': 6, 'T': 8}
Expected Output:
{'T': 1, 'C': 1, 'A': 1, 'G': 1}
{'T': 2, 'C': 1, 'A': 0, 'G': 1}
{'T': 2, 'C': 0, 'A': 0, 'G': 2}
{'T': 2, 'C': 1, 'A': 0, 'G': 1}
{'T': 1, 'C': 1, 'A': 1, 'G': 1}
However, in my function, as one can see, the dictionary is the same for all the windows and the nucleotide counts to the same dictionary key in every iteration. For every window (chunk) there should be different dictionary.

Answer 1

You should initialize d inside the loop instead so that it starts with a new dict for each iteration:
for i in range(len(s) - 3):
d = {}
...
If you want the dicts in the output to always have the same keys even if their values are 0, as suggested by your expected output, you can initialize a dict with all of the distinct letters as keys, and copy the dict to d for each iteration:
initialized_dict = dict.fromkeys(s, 0)
for i in range(len(s) - 3):
d = initialized_dict.copy()
...