Question

There is supposed to be a comma between these two strings

foo = ['dumb'
'error']

But if you forget the comma, it just merges the strings together instead of producing a syntax error. Your result will be

['dumberror']

I spent hours tracking this down. Why is the Python interpreter merging these strings?

Answer 1

It’s a documented feature to allow for nicer formatting in source code with string literals.

Multiple adjacent string or bytes literals (delimited by whitespace), possibly using different quoting conventions, are allowed, and their meaning is the same as their concatenation. Thus, “hello” ‘world’ is equivalent to “helloworld”. This feature can be used to reduce the number of backslashes needed, to split long strings conveniently across long lines, or even to add comments to parts of strings.

Source

It is also worth remembering this note:

Note that this feature is defined at the syntactical level, but implemented at compile time. The ‘+’ operator must be used to concatenate string expressions at run time. Also note that literal concatenation can use different quoting styles for each component (even mixing raw strings and triple quoted strings).