用索引替换python列表中的项目..失败？

Question

Any idea why when I call:

>>> hi = [1, 2]
>>> hi[1]=3
>>> print hi
[1, 3]

I can update a list item by its index, but when I call:

>>> phrase = "hello"
>>> for item in "123":
>>>     list(phrase)[int(item)] = list(phrase)[int(item)].upper()
>>> print phrase
hello

It fails?

Should be hELLo

Answer 1

You haven't initialised phrase (The list you were intending to make) into a variable yet. So pretty much you have created a list in each loop, it being the exact same.

If you were intending to actually change the characters of phrase , well that's not possible, as in python, strings are immutable.

Perhaps make phraselist = list(phrase) , then edit the list in the for-loop. Also, you can use range() :

>>> phrase = "hello"
>>> phraselist = list(phrase)
>>> for i in range(1,4):
...     phraselist[i] = phraselist[i].upper()
... 
>>> print ''.join(phraselist)
hELLo

Answer 2

>>> phrase = "hello"
>>> list_phrase = list(phrase)
>>> for index in (1, 2, 3):
        list_phrase[index] = phrase[index].upper()
>>> ''.join(list_phrase)
'hELLo'

If you prefer one-liner:

>>> ''.join(x.upper() if index in (1, 2, 3) else x for
            index, x in enumerate(phrase))
'hELLo'

Answer 3

Another answer, just for fun :)

phrase = 'hello'
func = lambda x: x[1].upper() if str(x[0]) in '123' else x[1]
print ''.join(map(func, enumerate(phrase)))
# hELLo

---

To make this robust, I created a method: (because I am awesome, and bored)

def method(phrase, indexes):
    func = lambda x: x[1].upper() if str(x[0]) in indexes else x[1]
    return ''.join(map(func, enumerate(phrase)))

print method('hello', '123')
# hELLo

Answer 4

认为字符串在python中是不可变的你不能修改现有的字符串可以创建新的。

''.join([c if i not in (1, 2, 3) else c.upper() for i, c in enumerate(phrase)])

Answer 5

list() creates a new list. Your loop creates and instantly discards two new lists on each iteration. You could write it as:

phrase = "hello"
L = list(phrase)
L[1:4] = phrase[1:4].upper()
print("".join(L))

Or without a list:

print("".join([phrase[:1], phrase[1:4].upper(), phrase[4:]]))

Strings are immutable in Python therefore to change it, you need to create a new string.

Or if you are dealing with bytestrings, you could use bytearray which is mutable:

phrase = bytearray(b"hello")
phrase[1:4] = phrase[1:4].upper()
print(phrase.decode())

If indexes are not consecutive; you could use an explicit for-loop:

indexes = [1, 2, 4]
for i in indexes:
    L[i] = L[i].upper()