字典理解：更改列表值中的最后一项

Question

I have a Python dictionary mapping strings to lists of strings.

• I want to change the last item of each list to uppercase using a dictionary comprehension

Eg for this example

dd = {'cc': ['aa', 'UU', 'zzzzzzz'], 't': ['aa', 'uu', 'ZZZZZZ']}

I want this:

{'cc': ['aa', 'UU', 'ZZZZZZZ'], 't': ['aa', 'uu', 'ZZZZZZ']} # note the last item in the first list 

I thought I could just re-assign the last item via indexing and tried variations of this

{k: (v[-1] = v[-1].upper()) for k,v in dd.viewitems()} 
# --> returns syntax error
# or this, which returns None:
{k: v[:-1].append(v[-1].upper()) for k,v in dd.viewitems()}
#the below works but I don't like it
{k: [i if not v.index(i) == len(v)-1 else i.upper() for i in v] for k,v in dd.viewitems()}

I can't get it to work w/out a dirty list comprehension.

• Is there a good way to do this?

(I am on Py 2 though through no fault of my own)

Thanks!

Answer 1

Use a simple for loop:

dd = {'cc': ['aa', 'UU', 'zzzzzzz'], 't': ['aa', 'uu', 'ZZZZZZ']}

for v in dd.itervalues():
    v[-1] = v[-1].upper()

print dd

{'cc': ['aa', 'UU', 'ZZZZZZZ'],
 't': ['aa', 'uu', 'ZZZZZZ']}

A dictionary comprehension cannot contain assignments, and so is inappropriate for the logic you are attempting to apply.

Answer 2

You can use unpacking:

dd = {'cc': ['aa', 'UU', 'zzzzzzz'], 't': ['aa', 'uu', 'ZZZZZZ']}
new_d = {a:[c, d, e.upper()] for a, [c, d, e] in dd.items()}

Output:

{'cc': ['aa', 'UU', 'ZZZZZZZ'], 't': ['aa', 'uu', 'ZZZZZZ']}

However, for lists with arbitrary numbers of elements, you can use negative indexing:

new_d = {a:b[:-1]+[b[-1].upper()] for a, b in dd.items()}