Mapping Names to Sequence Elements

Problem

You have code that accesses list or tuple elements by position, but this makes the code somewhat difficult to read at times. You’d also like to be less dependent on position in the structure, by accessing the elements by name.

Solution

collections.namedtuple() provides these benefits, while adding minimal overhead over using a normal tuple object.



In [1]:

    
from collections import namedtuple

Subscriber = namedtuple('Subscriber', ['addr', 'joined'])
sub = Subscriber('jonesy@example.com', '2012-10-19')
sub









    Out[1]:





Subscriber(addr='jonesy@example.com', joined='2012-10-19')



In [2]:

    
print(sub.addr)
print(sub.joined)









    



jonesy@example.com
2012-10-19

namedtuple is interchangeable with a tuple and supports all of the usual tuple operations such as indexing and unpacking.



In [3]:

    
print(len(sub))
addr, joined = sub
print(addr)
print(joined)









    



2
jonesy@example.com
2012-10-19

A major use case for named tuples is decoupling your code from the position of the elements it manipulates. So, if you get back a large list of tuples from a database call, then manipulate them by accessing the positional elements, your code could break if, say, you added a new column to your table. Not so if you first cast the returned tuples to namedtuples.

Here is the code using ordinary tuples



In [5]:

    
def compute_cost(records): 
    total = 0.0
    for rec in records:
        total += rec[1] * rec[2]
    return total

Here is a version that uses a namedtuple



In [6]:

    
from collections import namedtuple

Stock = namedtuple('Stock', ['name', 'shares', 'price'])

def compute_cost(records):
    total = 0.0
    for rec in records:
        s = Stock(*rec)
        total += s.shares * s.price
    return total

# Some Data
records = [
    ('GOOG', 100, 490.1),
    ('ACME', 100, 123.45),
    ('IBM', 50, 91.15)
]

print(compute_cost(records))

Discussion

Be aware that unlike a dictionary, a namedtuple is immutable. If you need to change any of the attributes, it can be done using the _replace() method of a namedtuple instance



In [8]:

    
s = Stock('ACME', 100, 123.45)
print(s)
s.shares = 75









    



Stock(name='ACME', shares=100, price=123.45)






    



---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-08525960b121> in <module>()
      1 s = Stock('ACME', 100, 123.45)
      2 print(s)
----> 3 s.shares = 75

AttributeError: can't set attribute



In [9]:

    
s = s._replace(shares=75)
s









    Out[9]:





Stock(name='ACME', shares=75, price=123.45)

We can also use _replace() method to populate named tuples that have optional or missing fields.



In [11]:

    
from collections import namedtuple
Stock = namedtuple('Stock', ['name', 'shares', 'price', 'date', 'time'])

# Create a prototype instance
stock_prototype = Stock('', 0, 0.0, None, None)

# Function to convert a dictionary to a Stock
def dict_to_stock(s):
    return stock_prototype._replace(**s)

a = {'name': 'ACME', 'shares': 100, 'price': 123.45}
dict_to_stock(a)









    Out[11]:





Stock(name='ACME', shares=100, price=123.45, date=None, time=None)



In [12]:

    
b = {'name': 'ACME', 'shares': 100, 'price': 123.45, 'date': '12/17/2012'}
dict_to_stock(b)









    Out[12]:





Stock(name='ACME', shares=100, price=123.45, date='12/17/2012', time=None)