Look up the value associated with the key:

When a key is not found, a Missing object is returned by default:

Provide a default value to be used when the key is not found:

Use the operator form of Lookup:

Look up multiple keys at once:

Lookup threads over lists of associations:

Query multiple keys from multiple associations:

Lookup can be used to look up keys in lists of rules:

Wrap the result in Hold to prevent it from evaluating:

Apply the operator form to a list of lists:

You can use constructs such as Throw and Return in the third argument of Lookup to abort a computation when a required key is missing:

When the key is present, the default is not evaluated:

Even when it appears in a result multiple times, the default value is only evaluated once:

Applying an association to a key is typically equivalent to using Lookup on that key:

Lists are handled differently by Lookup, however:

Wrap the list in Key to make Lookup treat it as a single key:

Lookup automatically strips up to one level of Key from its second argument:

Similarly, lists of keys are stripped of any Key wrappers they contain:

This stripping allows looking up expressions of the form Key[{…}] and Key[Key[…]]:

When the key to be looked up is a list or an expression with head Key, it must be wrapped in a Key to avoid ambiguity: