2.1.6 Mapping Types

















Python Library Reference




Previous: 2.1.5.4 Mutable Sequence Types
Up: 2.1 Built-in Types
Next: 2.1.7 Other Built-in Types





2.1.6 Mapping Types




A mapping object maps values of one type (the key type) to
arbitrary objects. Mappings are mutable objects. There is currently
only one standard mapping type, the dictionary. A dictionary's keys are
almost arbitrary values. The only types of values not acceptable as
keys are values containing lists or dictionaries or other mutable
types that are compared by value rather than by object identity.
Numeric types used for keys obey the normal rules for numeric
comparison: if two numbers compare equal (e.g. 1 and
1.0) then they can be used interchangeably to index the same
dictionary entry.


Dictionaries are created by placing a comma-separated list of
key: value pairs within braces, for example:
{'jack': 4098, 'sjoerd': 4127} or
{4098: 'jack', 4127: 'sjoerd'}.


The following operations are defined on mappings (where a and
b are mappings, k is a key, and v and x are
arbitrary objects):







Operation 
Result 
Notes 


len(a)
the number of items in a
 
a[k]
the item of a with key k
(1)
a[k] = v
set a[k] to v
 
del a[k]
remove a[k] from a
(1)
a.clear()
remove all items from a
 
a.copy()
a (shallow) copy of a
 
a.has_key(k)
1 if a has a key k, else 0
 
a.items()
a copy of a's list of (key, value) pairs
(2)
a.keys()
a copy of a's list of keys
(2)
a.update(b)
for k in b.keys(): a[k] = b[k]
(3)
a.values()
a copy of a's list of values
(2)
a.get(k[, x])
a[k] if a.has_key(k),
else x
(4)
a.setdefault(k[, x])
a[k] if a.has_key(k),
else x (also setting it)
(5)



Notes:

(1)
Raises a KeyError exception if k is not
in the map.



(2)
Keys and values are listed in random order. If
keys() and values() are called with no intervening
modifications to the dictionary, the two lists will directly
correspond. This allows the creation of (value,
key) pairs using map(): "pairs = map(None,
a.values(), a.keys())".



(3)
b must be of the same type as a.



(4)
Never raises an exception if k is not in the map,
instead it returns x. x is optional; when x is not
provided and k is not in the map, None is returned.



(5)
setdefault() is like get(), except
that if k is missing, x is both returned and inserted into
the dictionary as the value of k.










Python Library Reference




Previous: 2.1.5.4 Mutable Sequence Types
Up: 2.1 Built-in Types
Next: 2.1.7 Other Built-in Types



See About this document... for information on suggesting changes.