使用 if no in == False 存储唯一字符串

Question

writing a simple code to store unique string into list . However if i use if var in list == False the string output = blank . However if i use If var not in list , I gotten the desired output.

please advise why if var in list == False does not work here.

bank = list()

while True:
      word = input('Enter a word,type"done"to finish')

      if word == 'done': break

      if word in bank == False: # this code does't work as intended
            bank.append(word)

bank.sort()
print(bank)

Answer 1

Its a bit subtle but its because of Comparison chaining . Comparitors < | > | == | >= | <= | != | is [not] | [not] in < | > | == | >= | <= | != | is [not] | [not] in < | > | == | >= | <= | != | is [not] | [not] in have equal prescience and are compared left to right. In addition, chaining of the operations is the same as adding an "and". The familiar example is:

x < y <= z is equivalent to x < y and y <= z .

The same holds for in . Your expression is the same as

if (word in bank) and (bank == False):
    ....

Your working example if word in bank: is the canonical way of writing the expression... no need to make it complicated!

Answer 2

@tdelaney gave the correct above. Here is how you would write it:

if word not in bank:
  bank.append()

Or even better use a set instead of a list.

bank = set()
while True:
  word = input('Enter a word,type"done"to finish')
  if word == 'done':
    break
  bank.add(word)

bank = list(bank)
bank.sort()
print(bank);

Answer 3

Your code doesn't work because it represents it like:

if word in (bank == False):

It's like the order of operations in math, so it takes it first processes whether bank is False , not processing first whether word is in bank , so if you use parenthesis, it would work:

if (word in bank) == False:

If you change it to the above ^, it would work.

But of course it's more recommended if you use:

if word not in bank: