Check for dict key existence, and modify it in one step.

[rodrigo]

Im using this construct a lot:

if dict.has_key(whatever):
dict[whatever] += delta
else:
dict[whatever] = 1

sometimes even nested:

if dict.has_key(whatever):
if dict[whatever].has_key(someother):
dict[whatever][someother] += delta
else:
dict[whatever][someother] = 1
else:
dict[whatever]={}
dict[whatever][someother] = 1

there must be a more compact, readable and less redundant way to do
this, no?

Thanks,

Rodrigo

[Bruno Desthuilliers]

rodrigo a écrit :

Im using this construct a lot:

if dict.has_key(whatever):

<ot>
Avoid using builtin names as identifiers (it shadows the builtin name).
</ot>

Unless you're using an old Python version, you'd be better using
if whatever in my_dict:
# do something here

dict[whatever] += delta
else:
dict[whatever] = 1

sometimes even nested:

if dict.has_key(whatever):
if dict[whatever].has_key(someother):
dict[whatever][someother] += delta
else:
dict[whatever][someother] = 1
else:
dict[whatever]={}
dict[whatever][someother] = 1

there must be a more compact, readable and less redundant way to do
this, no?

There are other ways, yes. With Python <= 2.4.x, you can use
dict.setdefault, with Python 2.5.x you can use a defaultdict (cf
http://docs.python.org/whatsnew/modules.html).

HTH

[Evan Klitzke]

On Tue, 2007-08-28 at 14:36 +0000, rodrigo wrote:

Im using this construct a lot:

if dict.has_key(whatever):
dict[whatever] += delta
else:
dict[whatever] = 1

In Python 2.5 there's a defaultdict class, otherwise you can subclass
dict like this:

class CountingDictionary(dict):
def increment(self, key, delta=1):
self[key] = self.get(key, 0) + delta

Hope this helps!

--
Evan Klitzke <ev**@yelp.com>

[rodrigo]

evan,

yes, it does help. Works like it should:

class CountingDictionary(dict):
def increment(self, key, delta=1):
self[key] = self.get(key, 0) + delta

d = CountingDictionary()
d.increment('cat')
d.increment('dog',72)
print d

>>{'dog': 72, 'cat': 1}

Thanks!

[rodrigo]

You're right of course, I was unclear. I wasn't using 'dict' to
override the dict clas, but just as a standin for the example (the
actual dictionary names are varied).

Thanks,

Rodriog

[Ben Finney]

rodrigo <ro********@gmail.comwrites:

Im using this construct a lot:

if dict.has_key(whatever):
dict[whatever] += delta
else:
dict[whatever] = 1

I'd prefer:

foo.setdefault(whatever, 0)
foo[whatever] += delta

sometimes even nested:

if dict.has_key(whatever):
if dict[whatever].has_key(someother):
dict[whatever][someother] += delta
else:
dict[whatever][someother] = 1
else:
dict[whatever]={}
dict[whatever][someother] = 1

foo.setdefault(whatever, {})
foo[whatever].setdefault(someother, 0)
foo[whatever] += delta

there must be a more compact, readable and less redundant way to do
this, no?

Hope that helps.

--
\ "I took a course in speed waiting. Now I can wait an hour in |
`\ only ten minutes." -- Steven Wright |
_o__) |
Ben Finney

[Ben Finney]

Ben Finney <bi****************@benfinney.id.auwrites:

foo.setdefault(whatever, {})
foo[whatever].setdefault(someother, 0)
foo[whatever] += delta

Should, of course, be:

foo.setdefault(whatever, {})
foo[whatever].setdefault(someother, 0)
foo[whatever][someother] += delta

--
\ "My house is made out of balsa wood, so when I want to scare |
`\ the neighborhood kids I lift it over my head and tell them to |
_o__) get out of my yard or I'll throw it at them." -- Steven Wright |
Ben Finney

[Dustan]

On Aug 28, 1:13 pm, rodrigo <[e-mail]:

evan,

yes, it does help. Works like it should:

class CountingDictionary(dict):
def increment(self, key, delta=1):
self[key] = self.get(key, 0) + delta

d = CountingDictionary()
d.increment('cat')
d.increment('dog',72)
print d

>{'dog': 72, 'cat': 1}

Thanks!

You responded to the answer that made the least use of already
existing recourses in python. Just letting you know.

[Bruno Desthuilliers]

rodrigo a écrit :

You're right of course, I was unclear. I wasn't using 'dict' to
override the dict clas, but just as a standin for the example (the
actual dictionary names are varied).

I guessed so, but Python's behaviour wrt/ naming can be somewhat
surprising for newcomers, and accidentally shadowing builtins (or not
builtins FWIW) names is a common pitfall. So better to warn newcomers
lurking here !-)