How to remove KeyError with if else conditioning in python - python-2.7

Question

If there is a character that is not in my key list, such as "X", how do I avoid it and continue without doing nothing to it? I am getting KeyError 'X' , cause there is a X in my sequence that I am looking at.
keys = ["A", "C", "D", "E"]
for char in keys:
counts[char] = 0
for line in gpcr:
if line.startswith(">"):
line = line.replace(' ','')
header = line.split()
number = header[0].split('|')
print "Id:",number[2]
continue
fo.write(number[2])
fo.write('\n')
for char in line.strip():
if char
counts[char] += 1
total = float(sum(counts.values()))
toReturn = ''
for key in keys:
aa_per = (counts[key]/total)*100
toReturn = toReturn + '%.2f'%aa_per + '%'+ '\t'
fo.write(number[1])
fo.write('\n')
fo.write(''.join(str(x) for x in toReturn))
fo.write('\n')
print toReturn
fo.close()

Answer 1

I am slightly confused by your question. I guess the problematic line is
aa_per = (counts[key]/total)*100
You can check for a KeyError by using a try block:
try:
aa_per = (counts[key]/total)*100
except KeyError:
aa_per = 0
I guess if the key doesn't occur the percentage should be 0 here.
In general try blocks are a powerful tool to check for exceptions or warnings. See also herehttps://docs.python.org/3/tutorial/errors.html

Answer 2

If you still want to count the number of occurrences of this X character, you can use defaultdict
from collections import defaultdict
counts = defaultdict(int)
counts will be an instance of a dictionary that instead of raising a KeyError, will return 0 if the key does not exist. This way you will be able to avoid the dictionary initialization altogether.
Update:
If you want to use if-else, I think it should be enough to do:
for char in line.strip():
if char in keys:
counts[char] += 1