字典理解，同时检查键是否已存在？

Question

I know you can check if a key exists using:

if key in mydict:
    ...

but I wish to somehow use this in a dictionary comprehension as I construct the dictionary.

For example:

mylist = [('a', 0.01), ('b', 0.02), ('c', 0.03), ('a', 0.04)]
mydict = {item[0]: item[1] for item in mylist if item[0] not in mydict else blah blah}

What's the best way to achieve this?

I need the else part also.

Edit: For clarification. In my case I actually need the sum of item[1] values for all items with a given item[0] value.

Answer 1

Python dict is unordered structure with unique keys. As you need values of firstly encountered keys - iterate input list in reversed order:

mylist = [('a', 0.01), ('b', 0.02), ('c', 0.03), ('a', 0.04)]
mydict = {t[0]:t[1] for t in mylist[::-1]}

print(mydict)

The output:

{'a': 0.01, 'c': 0.03, 'b': 0.02}

Answer 2

With dictionary comprehension:

mylist = [('a', 0.01), ('b', 0.02), ('c', 0.03), ('a', 0.04)]
mydict = {key:value for key, value in mylist[::-1]}
print(mydict)

Output:

{'a': 0.01, 'c': 0.03, 'b': 0.02}

Without dictionary comprehension:

mydict = {}
for key, value in mylist:
    if key not in mydict:
        mydict[key] = value
    else:
        # You asked for the else part. Do whatever here.   
print(d)

Output:

{'a': 0.01, 'c': 0.03, 'b': 0.02}