Python concatenates adjacent string or byte literals at compile time. It means that "a" "b" is equivalent to "ab". This is sometimes used to split a long string on multiple lines. However an implicit string concatenation can also be very confusing. In the following contexts it might indicate that a comma was forgotten:

Noncompliant Code Example

def func():
    return "item1" "item2"  # Noncompliant

["1",
 "2"  # Noncompliant
 "3",
 "a very very very"  # Noncompliant
 "very very long string",
 "4"]

Compliant Solution

def func():
    return "item1", "item2"

["1",
 "2",
 "3",
 "a very very very" +
 "very very long string",
 "4"]

Exceptions

No issue will be raised when there is a visible reason for the string concatenation: