Answered by Martijn Pieters. Thank you.
It is because statement vs expression.
And because .join() does not mutate (is a pure function), so it needs to be assigned to a variable.
Question:
What is the reason for this oddity?
Goal:
if base == 'T':
RNA_seq += 'U'
else:
RNA_seq += base
This following method works:
# += in expression1 and .join() in expression2
RNA_seq += 'U' if base == 'T' else RNA_seq.join(base)
# Edit note: RNA_seq.join(base) works because it returns `base`
# aka. RNA_seq += 'U' if base == 'T' else base
However the following does NOT work:
# Using += on both expressions
RNA_seq += 'U' if base == 'T' else RNA_seq += base
or
# Using .join() for both expressions
RNA_seq.join('U') if base == 'T' else RNA_seq.join(base)
The results are the same in both Python2 and Python3.
Like all assignment, += is a statement. You cannot ever put statements in an expression. The right-hand expression (everything after +=) is evaluated first, the result of which is then used for the augmented assignment.
You can do:
RNA_seq += 'U' if base == 'T' else base
Now the expression resolves either to 'U', or base, depending on the value of base.
If 'U' if base == 'T' else RNA_seq.join(base) worked, then that means that RNA_seq.join() returns a new value and doesn't mutate RNA_seq in-place. RNA_seq.join('U') if base == 'T' else RNA_seq.join(base) would then also return a new value, leaving the original value bound by RNA_seq unchanged, and you didn't assign it back to RNA_seq.