if (cdr3_length(b[i]) > 15 & heavy_percent(b[i]) < 88
candidates.append(b[i])
->
if (cdr3_length(b[i]) > 15) and (heavy_percent(b[i]) < 88):
candidates.append(b[i])
python uses indentation to form blocks of code, so you should watch this closely. Also you may want to visit python.org an read its awesome beginners guide. While both typing an syntax are both relaxed in this language they still demand respect
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