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
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