How Can I Merge Two Python Dictionaries and Combine Values for Duplicate Keys?

[ver_mathstats]

Homework Statement

Write a Python code to concatenate two dictionaries; if the two dictionaries have the same key, then the values corresponding to this key will be
appended.

Relevant Equations

Python

dict1={"a":1,"b":2}
dict2={"a":3, "d":4}
for k in dict2:
    if k in dict1:
        dict2[k] = dict1.append(dict2)
dict1.update(dict2)
print(dict1)

I need the result to be {'a': [1, 3], 'b': 2, 'd': 4} and I am confused as to how to achieve this as I always get an error message. I've tried this as well dict2[k] = dict2[k].append(dict1[k]). Any help is appreciated, thank you.

 

[scottdave]

You need to make that dictionary entry be a list, then append the new value to that list. In your if statement, you would want to check to see if the entry is already a list - if that is a concern that the dictionary already contains some list values as well as integer values.

 

[Nick-stg]

Python dictionaries have several methods to simplify looping.

for key in my_dict:
     print(key) #prints the keys of the items in the dictionary a,b,c for your dict1

for value in my_dict.values():
     print(value) #prints the values of the items in the dictionary 1,2,3 for your dict1

for key,value in my_dict.items():
     print(key, value) #prints the values of the items in the dictionary a 1, b 2, b 3 for your dict1

Once you have are able to loop through the dictionary you then need to check if the key exists. To do that:

#comparing each key to some string or strings
for key,value in my_dict.items():
     if key == 'a': 
          print(key, value) #prints a 1, for your dict1

#Or:
# a string to all the keys
if 'a' in my_dict:
      print(key, value) #prints a 1, for your dict1

Finally you need to treat the case where the key exists differently to where the key doesn't exist.

# if the key exists you need to "add" the value to that item eg: append if the value is a list.
my_dict[key].append('add something new')
# if the key doesn't exists you need to "create" new item. Remember to create a list if you need to add many elements.
my_dict[key] = ['something new']

Note that a dictionary value can include all kinds of datatypes, string, list, dict, etc. So you may need to check the datatype before you do anything else. For example in your case adding elements from dict2 to dict1, you can't append a string to a string or int to an int. You will need to make "a" a list with the value from dict 1 and then append the value of dict 2. In which case it is likely easier to create a 3rd new dict of lists.

Side note to regular members and/or mods: I'm new to this answering homework help. I'm trying to provide explanation without providing the answers directly, I am assuming that this is the goal here?