Python: collections.namedtuple

Classes com criação dinâmica.

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Python Module of the Week

3-4 minutos

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The standard tuple uses numerical indexes to access its members.

bob = ('Bob', 30, 'male')
print 'Representation:', bob

jane = ('Jane', 29, 'female')
print '\nField by index:', jane[0]

print '\nFields by index:'
for p in [ bob, jane ]:
    print '%s is a %d year old %s' % p

This makes tuples convenient containers for simple uses.

$ python collections_tuple.py

Representation: ('Bob', 30, 'male')

Field by index: Jane

Fields by index:
Bob is a 30 year old male
Jane is a 29 year old female

On the other hand, remembering which index should be used for each value can lead to errors, especially if the tuple has a lot of fields and is constructed far from where it is used. A namedtuple assigns names, as well as the numerical index, to each member.

Defining¶

namedtuple instances are just as memory efficient as regular tuples because they do not have per-instance dictionaries. Each kind of namedtuple is represented by its own class, created by using the namedtuple() factory function. The arguments are the name of the new class and a string containing the names of the elements.

import collections

Person = collections.namedtuple('Person', 'name age gender')

print 'Type of Person:', type(Person)

bob = Person(name='Bob', age=30, gender='male')
print '\nRepresentation:', bob

jane = Person(name='Jane', age=29, gender='female')
print '\nField by name:', jane.name

print '\nFields by index:'
for p in [ bob, jane ]:
    print '%s is a %d year old %s' % p
    

As the example illustrates, it is possible to access the fields of the namedtuple by name using dotted notation (obj.attr) as well as using the positional indexes of standard tuples.

$ python collections_namedtuple_person.py

Type of Person: <type 'type'>

Representation: Person(name='Bob', age=30, gender='male')

Field by name: Jane

Fields by index:
Bob is a 30 year old male
Jane is a 29 year old female

Invalid Field Names¶

As the field names are parsed, invalid values cause ValueError exceptions.

import collections

try:
    collections.namedtuple('Person', 'name class age gender')
except ValueError, err:
    print err

try:
    collections.namedtuple('Person', 'name age gender age')
except ValueError, err:
    print err
    

Names are invalid if they are repeated or conflict with Python keywords.

$ python collections_namedtuple_bad_fields.py

Type names and field names cannot be a keyword: 'class'
Encountered duplicate field name: 'age'

In situations where a namedtuple is being created based on values outside of the control of the programm (such as to represent the rows returned by a database query, where the schema is not known in advance), set the rename option to True so the fields are renamed.

import collections

with_class = collections.namedtuple('Person', 'name class age gender', rename=True)
print with_class._fields

two_ages = collections.namedtuple('Person', 'name age gender age', rename=True)
print two_ages._fields

The field with name class becomes _1 and the duplicate age field is changed to _3.

$ python collections_namedtuple_rename.py

('name', '_1', 'age', 'gender')
('name', 'age', 'gender', '_3')