Append a new value to dict

[Mathias Frey]

Pat wrote:
> I know it's not "fair" to compare language features, but it seems to me 
> (a Python newbie) that appending a new key/value to a dict in Python is 
> awfully cumbersome.
> 
> In Python, this is the best code I could come up with for adding a new 
> key, value to a dict
> 
> mytable.setdefault( k, [] ).append( v )
> 
> In Perl, the code looks like this:
> 
> $h{ $key } = $value ;

There's a huge difference here:

In your Python example you're using a list. In the Perl example you're 
using a scalar value.


> Is there a better/easier way to code this in Python than the 
> obtuse/arcane setdefault code?

When just assigning a new key-value-pair there's no problem in Python. 
(Just refer to the answers before.) When I switched from Perl to Python 
however I commonly ran into this problem:

 >>> counter = {}
 >>> counter['A'] = 1
 >>> counter['A'] += 1
 >>> counter['A']
2

Ok - assigning a key-value-pair works fine. Incrementing works as well.

 >>> counter['B'] += 1
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: 'B'

However incrementing a non-existing key throws an exception. So you 
either have to use a workaround:

 >>> try:
...   counter['B'] += 1
... except KeyError:
...   counter['B'] = 1

Since this looks ugly somebody invented the setdefault method:

 >>> counter['B'] = counter.setdefault('B',0) + 1

And this works with lists/arrays as well. When there's no list yet 
setdefault will create an empty list and append the first value. 
Otherwise it will just append.

Greetings from Vienna,
mathias