if key in data_dict.keys():
    add values
else:
    data_dict[key] = value

try:
    # This will fail if key not present
    data_dict[keyStr] = input_data[keyStr] + load_val
except:
    data_dict[keyStr] = load_val

if key in data_dict:

from collections import defaultdict

data_dict = defaultdict(list)
for keyStr, load_val in data:
    data_dict[keyStr].append(load_val)

if key in data_dict:

if key in data_dict.keys():

In [258]: data_dict = dict([(x, x) for x in range(100000)])

In [259]: %timeit 999999 in data_dict.keys()
100 loops, best of 3: 3.47 ms per loop

In [260]: %timeit 999999 in data_dict
10000000 loops, best of 3: 49.3 ns per loop

from collections import defaultdict

input_data = defaultdict(float)  # (guessing factory type)
...
data_dict[keyStr] = input_data[keyStr] + load_val

data_dict.setdefault(keyStr, []).append(load_val)

data_dict[keyStr] = data_dict.get(keyStr, '') + load_val