Pandas check if two conditions are true and change row

Question

I have a pandas dataframe with four columns: ['Source', 'Target', 'Type', 'Weight']

The combination of 'Source' and 'Weight' is basically the key for the dataframe, so at most one combination of 'Source' and 'Weight' is possible.

Now I want to check, whether a certain row is already in the dataframe, if so, access it (by adding +1 to Weight) and if not, create it.

How can I search for two conditions, access if there is a match and create a new row if not?

    if(df[(df.Source == source) & (df.Target == target)].empty()):
        df = df.append([source, target, 'bidirectional', 1])
    else:
        df[(df.Source == source) & (df.Target == target)].Weight += 1