Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Sparse assignment list in python

Tags:

python

I need a list with the following behavior

>>> l = SparseList()
>>> l
[]
>>> l[2] = "hello"
>>> l
[ None, None, "hello"]
>>> l[5]
None
>>> l[4] = 22
>>> l
[ None, None, "hello", None, 22]
>>> len(l)
5
>>> for i in l: print i
None
None
"hello"
None
22

Although it can "emulated" via a dictionary, it's not exactly the same. numpy array can behave this way, but I don't want to import the whole numpy for something like this. Before coding it myself, I ask if something similar exists in the standard library.

like image 717
Stefano Borini Avatar asked Dec 07 '09 04:12

Stefano Borini


1 Answers

Here's minimal code to pass your given examples (with indispensable adjustments: you expect weird spacing and quoting, 'None' to be printed out at the prompt without a print statement, etc etc):

class SparseList(list):
  def __setitem__(self, index, value):
    missing = index - len(self) + 1
    if missing > 0:
      self.extend([None] * missing)
    list.__setitem__(self, index, value)
  def __getitem__(self, index):
    try: return list.__getitem__(self, index)
    except IndexError: return None

__test__ = dict(allem='''
>>> l = SparseList()
>>> l
[]
>>> l[2] = "hello"
>>> l
[None, None, 'hello']
>>> print l[5]
None
>>> l[4] = 22
>>> l
[None, None, 'hello', None, 22]
>>> len(l)
5
>>> for i in l: print i
None
None
hello
None
22
''')
import doctest
doctest.testmod(verbose=1)

I imagine you'll want more (to support negative indices, slicing, and whatever else), but this is all your examples are implicitly specifying.

like image 60
Alex Martelli Avatar answered Sep 30 '22 04:09

Alex Martelli