Add a checker warning against string concatenation by juxtaposition

[nathanielmanistaatgoogle]

String concatenation by juxtaposition is bad both in theory and practice. Pylint should when seeing it warn the user and suggest explicit concatenation using the + operator.

Philosophy: String concatenation by juxtaposition breaks referential transparency

A foundational principle of computer science and programming languages is referential transparency, which states that for expressions that do not have side effects, replacing one expression that evaluates to a given value by another expression that evaluates to the same value has no effect on program behavior. Were referential transparency respected

my_string = 'abc' 'xyz'

and

my_prefix = 'abc'
my_string = my_prefix 'xyz'

would assign the same value to my_string. Because string-concatenation-by-juxtaposition is a language misfeature that violates referential transparency, the former code assigns the value 'abcxyz' to my_string and the latter code raises a SyntaxError. Yuck.

Pragmatism: String concatenation by juxtaposition fosters defects

I don't know a Python programmer who hasn't at some point accidentally written some form of

_MY_FIXED_VALUES = (
    'abcd',
    'efghi',
    'jk'
    'l',
    'mno',
)

and then (depending on the circumstances) taken some time to find the bug.

[PCManticore]

Hey @nathanielmanistaatgoogle Thanks for creating the issue. We already have this check under the name of implicit-str-concat-in-sequence. Unfortunately it's specific to sequences, but I could definitely see it useful for the assignment case as well. For multi-line jumps you'd have to run pylint with --check-str-concat-over-line-jumps=y, since by default it looks for occurrences on the same line.

[PCManticore]

Thanks for the report, this is fixed now in master and will be part of 2.5. For multiline juxtaposition, you need to pass --check-str-concat-over-line-jumps=y or set it in the configuration file.