defaultdict of fixed-length buffers



In [1]:

    
from collections import deque



In [2]:

    
# Buffer with maximum length of 3
buffer = deque([], 3)



In [3]:

    
buffer









    Out[3]:





deque([])



In [4]:

    
buffer.append(1)



In [5]:

    
buffer









    Out[5]:





deque([1])



In [7]:

    
buffer.append(2)
buffer.append(3)



In [8]:

    
buffer









    Out[8]:





deque([1, 2, 3])



In [9]:

    
buffer.append(4)



In [10]:

    
# First item is kicked out
buffer









    Out[10]:





deque([2, 3, 4])

Now let's say we want a number of these buffers that we want to reference via a dictionary

`defaultdict`



In [11]:

    
from collections import defaultdict

defaultdict takes a callable that'll be used as the default value if the requested key doesn't exist in the dict



In [12]:

    
x = defaultdict(list)



In [15]:

    
x['a']









    Out[15]:





[]



In [16]:

    
x['b'].append(2)



In [17]:

    
x









    Out[17]:





defaultdict(list, {'a': [], 'b': [2]})

The default value is generated upon the first access

`partial`

partial allows you to partially apply a function. In our case it's nice since we can create a callable for defaultdict.



In [19]:

    
from functools import partial



In [22]:

    
x = partial(deque, maxlen=3)
x









    Out[22]:





functools.partial(<class 'collections.deque'>, maxlen=3)



In [23]:

    
x()









    Out[23]:





deque([])

Putting the two together



In [24]:

    
x = defaultdict(partial(deque, maxlen=3))



In [25]:

    
x









    Out[25]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3), {})



In [26]:

    
x['a']









    Out[26]:





deque([])



In [27]:

    
x









    Out[27]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3),
            {'a': deque([])})



In [28]:

    
x['a'].append(1)
x['a'].append(2)



In [29]:

    
x









    Out[29]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3),
            {'a': deque([1, 2])})



In [30]:

    
x['a'].append(3)
x['a'].append(4)



In [31]:

    
x









    Out[31]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3),
            {'a': deque([2, 3, 4])})



In [32]:

    
x['b']









    Out[32]:





deque([])



In [33]:

    
x['b'].append(1)
x['b'].append(2)
x['b'].append(3)
x









    Out[33]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3),
            {'a': deque([2, 3, 4]), 'b': deque([1, 2, 3])})



In [34]:

    
x['b'].append(4)



In [35]:

    
x









    Out[35]:





defaultdict(functools.partial(<class 'collections.deque'>, maxlen=3),
            {'a': deque([2, 3, 4]), 'b': deque([2, 3, 4])})