Python Dictionaries
Contents
Use dict.get(x) and not dict[x]
>>> a = {"name": "sandipb", "age": 16}
>>> print a["height"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'height'
>>> print a.get("height")
None
>>> HEIGHT_UNSET = "0"
>>> print a.get("height", HEIGHT_UNSET)
0
>>> if a.get("height", HEIGHT_UNSET) == HEIGHT_UNSET:
... print "height not set"
...
height not set
>>> if not a["height"]:
... print "height not set"
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'height'
Use dict.setdefault(k,v) when you are building a dict with mutable values, and want to ensure that the entries have a default value before you start modifying it
Here is a program which gives you the frequency of alphabets in a string, and its positions.
1 import sys
2
3 if len(sys.argv) > 1:
4 word = " ".join(sys.argv[1:])
5 else:
6 word = "A big hello to the entire world"
7
8 # Storing positions of the alpha in the string
9 alphacount = {}
10
11 for pos,c in enumerate(word):
12 # print "[" + str(pos) + "] = ", c
13 if c in alphacount:
14 alphacount[c].append(pos)
15 else:
16 alphacount[c] = [] #Understand this is not required,
17 # Just that some languages require
18 # you to do something similar. e.g.
19 # malloc() if doing this in C
20 alphacount[c].append( pos)
21
22 print "word is [" + word + "]"
23 print "word is [" + "".join( map(str, (x % 10 for x in range(len(word))))) + "]"
24
25 for c in alphacount:
26 print "%s: %d times, positions %s" % ( c,
27 len(alphacount[c]),
28 ",".join(map(str, alphacount[c])))
29
$ python newdict.py hello there
word is [hello there]
word is [01234567890]
: 1 times, positions 5
e: 3 times, positions 1,8,10
h: 2 times, positions 0,7
l: 2 times, positions 2,3
o: 1 times, positions 4
r: 1 times, positions 9
t: 1 times, positions 6
The dict buildup here has to check if a key exists, if not, then has to create an empty list, and only then do a modification of the list by adding the line position into it.
It is this kind of logic which is solved using dict.setdefault(). In this case, that whole conditional (line 11 - 20 above ) can be replaced with:
1 for pos,c in enumerate(word):
2 alphacount.setdefault(c, []).append(pos)
3
* Reference: Python cookbook entry