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Dictionaries in Python: look things up by key

Some data isn't a line-up — it's a lookup. A dictionary stores pairs, so you can ask for a value by its key, like reading an amino acid straight from its codon.

The big idea

A dictionary maps keys to values, letting you fetch a value instantly by its key instead of its position.

See it in code

1The basics

A dictionary stores key: value pairs in { }. Here two codons map to their amino acids, and codon_table["AUG"] fetches one straight by its key:

python
codon_table = {"AUG": "Met", "UUU": "Phe"}
print(codon_table["AUG"])
Run it — the key jumps straight to its value:
Met

No searching, no index — ["AUG"] went right to "Met". That instant lookup is the whole point.

2A step further

len() counts the pairs, and any key you know fetches its value. Same table, two questions asked of it:

python
codon_table = {"AUG": "Met", "UUU": "Phe"}
print("Codons known:", len(codon_table))
print("UUU codes for:", codon_table["UUU"])
Run it — the count, then a second lookup:
Codons known: 2
UUU codes for: Phe

Two pairs, and ["UUU"] reached the second one directly. Now let's grow the table and loop over several codons.

3In our world

In biology, each three-letter codon codes for an amino acid — a perfect dictionary. We add GGC, then loop a list of codons, looking each one up by key:

python
codon_table = {
    "AUG": "Met",
    "UUU": "Phe",
    "GGC": "Gly",
}

print("AUG codes for:", codon_table["AUG"])
print("Codons known:", len(codon_table))

for codon in ["AUG", "GGC"]:
    print(codon, "->", codon_table[codon])
Run it — each key maps straight to its value:
AUG codes for: Met
Codons known: 3
AUG -> Met
GGC -> Gly

A list would make you search for the right pair; the dictionary jumps to it by key in one step — even inside a loop. Keys are unique and must be immutable (strings and numbers work); values can be anything.

The same idea, everywhere

Dictionaries model any 'look up X by Y': a player's stats by name, a config of settings, word counts in a text, a cache of results. When your data is labeled rather than ordered, reach for a dictionary — the lookup stays fast even with millions of keys.

Try it yourself

Add "UAA": "Stop" to the table, then look it up. Next, guard a missing key with codon_table.get("XYZ", "unknown"), and loop the pairs with for k, v in codon_table.items():.

The common mistake

Indexing a key that isn't there. codon_table["XYZ"] raises KeyError and stops the program. When a key might be missing, use .get(key, default), which returns your fallback instead of crashing.

What it unlocks

Key-value lookups power the genetic code and transcription and translation, and complement ordered lists.

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