TypeError when sum value of dictionnary for each key

I want to sum one value that contains list inside value dictionary.

I want to sum value of key 'x1' in dataX dictionary that contains list ([12, 14, 10, 16, 18, 24, 12]) but it goes wrong and an error appears.

Here is my dictionary:

dataX = {'x1' : [12, 14, 10, 16, 18, 24, 12], 'x2' : [10, 11, 14, 13, 15, 20, 8]}

the output should :

106

Here is my code:

sum = 0
for i in dataX:
    sum += dataX[i]

But, I get an error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/s6/_5szgcrd2cl86md1wj_kw_th0000gp/T/ipykernel_18467/658532054.py in <module>
      9 sum = 0
     10 for i in dataX:
---> 11     sum += dataX[i]

TypeError: unsupported operand type(s) for +=: 'int' and 'list'

Use the built-in sum() function

You need to use the sum built-in function, but be careful, you are doing name shadowing with the variable sum.

In programming, shadowing refers to the act of reusing the same name for a variable or function (here, sum) in an inner scope (your file), hiding its definition in an outer scope (built-ins). This can result in the outer definition being temporarily inaccessible (you can't use sum in your file), because the inner definition takes precedence (your sum variable, an int).

> ⚠️ If you are using Jupyter Notebook or an interactive
> session, please note that you need to restart the python session
> even if you changed the variable in the code, because sum has
> changed in the memory frame of the session. You need to restart Python
> to restore the sum function.

If you want the sum of each values:

sum_dict = {}
for key in dataX:
    sum_dict[key] = sum(dataX[key])

print(sum_dict)
# {'x1': 106, 'x2': 91}

Some improvements

iterate over key-value pairs

dict.items() allows you to iterate over the dictionnary with pairs of key and value together, which make manipulation of data less boilerplate.

sum_dict = {}
for key, values in dataX.items():
    sum_dict[key] = sum(values)

print(sum_dict)
# {'x1': 106, 'x2': 91}

Use dict comprehension

If you want a dict with keys and total for each list, a dict comprehension is fine since this operation is simple and can be expressed like this without reducing readability.

sum_dict = {key: sum(values) for key, values in dataX.items()}

print(sum_dict)
# {'x1': 106, 'x2': 91}

There is one more issue in your code;

You are using the builtin sum as your variable name.

It will override the builtin function and will not add values. sum will behave as an integer.

It is strongly advised not to use builtin function names like list, sum, etc. as your variable names.

you can get the sum of value by:

dataX = {'x1' : [12, 14, 10, 16, 18, 24, 12], 'x2' : [10, 11, 14, 13, 15, 20, 8]}
sum(dataX['x1'])
#106

or

for i in dataX:
    print(sum(dataX[i]))
106
91