当 IF 语句为假时为变量赋值会导致 Python 中的错误

Question

I'm writing a function in Python that should search a string for a substring, then assign the index of the substring to a variable. And if the substring isn't found, I'd like to assign -1 to the variable to be used as a stop code elsewhere. But I get an error that I don't understand. Here is a simplified version of the code:

test = "abc"
search_str = "z"
index_search_str = test.index(search_str) if search_str in test else index_search_str = -1

If I run this code, the value of index_search_str should be -1, but instead I get this error (using PyCharm):

 index_search_str = test.index(search_str) if search_str in test else index_search_str = -1
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

But if I change = -1 to := -1 , it still gives an error. What am I missing?

Answer 1

I think, in using ternary operator, value should be returned.

test = "azbc"
search_str = "z"
index_search_str = test.index(search_str) if search_str in test else -1

print(index_search_str)    # print value maybe  "1"

Answer 2

You cannot assign variable in one-line statement

test = "abc"
search_str = "z"
index_search_str = test.index(search_str) if search_str in test else -1

Answer 3

Your code have syntax errors.

I think you need something like this:

test = "abc"
search_str = "z"
if search_str in test:
    print("match")
    index_search_str = test.index(search_str)
    print(index_search_str)
else :
    print("not match")
    index_search_str = -1
    print(index_search_str)

"not match"
"-1"

test = "abc"
search_str = "c"
if search_str in test:
    print("match")
    index_search_str = test.index(search_str)
    print(index_search_str)
else :
    print("not match")
    index_search_str = -1
    print(index_search_str)

match
2

Answer 4

Just use str.find instead... it does exactly what you're trying to do by default.

>>> test = "abc"
>>> print(test.find('z'))
-1

>>> print(test.find('b'))
1

Answer 5

Try

index_search_str = test.index(search_str) if search_str in test else -1