如何在不覆盖的情况下将新值附加到现有的dict键

Question

The title describes the problem, this is what i tried and this is not giving me expected resoult.. where is the problem?

for name in file:              
  if name in list:
    if dict.has_key(param):
      dict[param] = [dict[param],name]
    else:
      dict[param] = name

expecting output:

dict = {
'param1': ['name11', 'name12', 'name13'],
'param2': ['name21', 'name22', 'name23'],
.
.
.
}

Answer 1

You need to append these names to a list. The following is a naive example and might break.

for name in file:              
    if name in list:
        if dict.has_key(param):
            dict[param].append(name)
        else:
            dict[param] = [name]

Or if you want to be a little cleaner, you can use collections.defaultdict . This is the pythonic way if you ask me.

d = collections.defaultdict(list)
for name in file:
    if name in lst: # previously overwrote list()
        d[param].append(name)

Please do not overwrite the builtin dict() or list() function with your own variable.

Answer 2

You are looking for a multi map, ie a map or dictionary which does not have a key -> value relation but a key -> many values, see:

See Is there a 'multimap' implementation in Python?