I recently read a post on planet python and the author mentioned something about
named tuple which made my curious.
So what's a named tuple?
The
namedtuple was introduced in Python 2.6. A named tuple is created using a factory function from the collections module and it extends the basic tuple by assigning a name to each position in a tuple but can still be used as a regular tuple. This makes it possible to access fields by name instead of an index. The named tuple should not require more memory, according to the documentation, than regular tuples since they don't have a per instance dictionary.
The factory function signature is:
collections.namedtuple(typename, field_names[, verbose])
The first argument specifies the name of the new type, the second argument is a string (space or comma separated) containing the field names and finally if
verbose is true the factory function will also print the class generated.
Enough theory, I'll show you an example.
Example
Say you have a tuple containing username and password. To access the username you get the item at position zero and the password is accessed at position one:
credential = ('mario', 'secret')
print 'Username:', credential[0]
print 'Password:', credential[1]
There's nothing wrong with this code but the tuple isn't self-documented. You have to find and read the documentation about the positioning of the fields in the tuple. This is where named tuple can enter the scene. We can rewrite the previous example as following:
import collections
# Create a new sub-tuple named Credential
Credential = collections.namedtuple('Credential', 'username, password')
credential = Credential(username='mario', password='secret')
print 'Username:', credential.username
print 'Password:', credential.password
Nice, don't you agree?
If you are interested of what the code looks like for the newly created
Credential-type you can add
verbose=True to the argument list when creating the type, in this particular case we get the following output:
import collections
Credential = collections.namedtuple('Credential', 'username, password', verbose=True)
class Credential(tuple):
'Credential(username, password)'
__slots__ = ()
_fields = ('username', 'password')
def __new__(_cls, username, password):
return _tuple.__new__(_cls, (username, password))
@classmethod
def _make(cls, iterable, new=tuple.__new__, len=len):
'Make a new Credential object from a sequence or iterable'
result = new(cls, iterable)
if len(result) != 2:
raise TypeError('Expected 2 arguments, got %d' % len(result))
return result
def __repr__(self):
return 'Credential(username=%r, password=%r)' % self
def _asdict(t):
'Return a new dict which maps field names to their values'
return {'username': t[0], 'password': t[1]}
def _replace(_self, **kwds):
'Return a new Credential object replacing specified fields with new values'
result = _self._make(map(kwds.pop, ('username', 'password'), _self))
if kwds:
raise ValueError('Got unexpected field names: %r' % kwds.keys())
return result
def __getnewargs__(self):
return tuple(self)
username = _property(_itemgetter(0))
password = _property(_itemgetter(1))
The named tuple doesn't only provide access to fields by name but also contains helper functions such as the
_make() function which helps creating an
Credential instance from a sequence or iterable. For example:
cred_tuple = ('mario', 'secret')
credential = Credential._make(cred_tuple)
There are more interesting use-cases and examples in the documentation, so I suggest that you take a peek.
Comments
I think the named tuple is useful. They remove the error-prone indexing in tuples by providing
access to fields by name without adding any memory overhead. They are also regular Python classes which means you can do anything
you can do with classes.
