Dictionaries

A dictionary maps keys to values. Use one whenever you need to look something up by name rather than by position.

depths = {"TP53": 120, "BRCA1": 88, "EGFR": 143}

print(depths["TP53"])
print(len(depths))

Reading safely

Indexing a missing key raises KeyError. get returns a default instead.

depths = {"TP53": 120}

print(depths.get("KRAS"))
print(depths.get("KRAS", 0))
print("KRAS" in depths)

Adding and updating

depths = {"TP53": 120}

depths["KRAS"] = 45
depths["TP53"] = 130
del depths["KRAS"]
print(depths)

Looping

depths = {"TP53": 120, "BRCA1": 88}

for gene in depths:
    print(gene)

for gene, depth in depths.items():
    print(gene, depth)

print(list(depths.keys()))
print(list(depths.values()))

Counting

A common pattern is counting occurrences. get with a default of 0 avoids a separate check for the first time a key appears.

sequence = "ATGGCC"
counts = {}
for base in sequence:
    counts[base] = counts.get(base, 0) + 1
print(counts)

Task

Write base_counts(sequence). Return a dictionary counting how many times each of A, C, G and T appears. Include all four keys even when the count is 0, and return them in the order A, C, G, T.