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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def subtract_by_key(dict_a, dict_b):
 """given two dicts, a and b, this function returns c = a - b, where a - b is defined as the key difference between a and b. e.g., {1:None, 2:3, 3:"yellow", 4:True} - {2:4, 1:"green"} = {3:"yellow", 4:True} """

    difference_dict = {}
    for key in dict_a:
        if key not in dict_b:
            difference_dict[key] = dict_a[key]

    return difference_dict