It has been pointed out to me that the errors the tokenizer produces for unterminated strings, "EOL while scanning string literal" and "EOF while scanning triple-quoted string literal", contain parsing jargon that make it difficult for new users to understand the problem, likely a missing quote.

It could be even better. Inside the tokenizer we know where the string literal starts and what quotes it uses. The line and the offset of the *start* of the literal can be set in a SyntaxError.

Here's my suggestion:

End of line reached without finding the end of string literal. Are you missing a closing quote?

Just re-posting this here from the open PR. Rust's handling of this seems nice and beginner friendly:

error: unterminated double quote string
--> src/main.rs:2:19
|
2 | let message = "Hello world
| __________________^
3 | | println!(message);
4 | | }
| |^

Like Serhiy suggested, it points to the /start/ of the string, rather than the EOL and the message seems nice too.

I afraid there may be confusion between triple, double and single quoted string literals. So I suggest to change error messages to just "unterminated triple-quoted string literal" and "unterminated string literal" (or "unterminated single-quoted string literal"). Terms "triple-quoted" and "single-quoted" are used several times in the documentation. Term "double-quoted" is used only once, and I suppose in different meaning.

Fair point. I changed error messages to what you suggested

>>> (300) * 6 + ca(e, 2 +    "dsadsa)
  File "<stdin>", line 1
    (300) * 6 + ca(e, 2 +    "dsadsa)
                             ^
SyntaxError: unterminated string literal

>>> (300) * 6 + ca(e, 2 +    'dsadsa)
  File "<stdin>", line 1
    (300) * 6 + ca(e, 2 +    'dsadsa)
                             ^
SyntaxError: unterminated string literal


>>> (300) * 6 + ca(e, 2 +    """dsadsa
... 
  File "<stdin>", line 1
    (300) * 6 + ca(e, 2 +    """dsadsa
                             ^
SyntaxError: unterminated triple-quoted string literal

New changeset 72e0aa2 by Batuhan Taskaya in branch 'master':
bpo-40176: Improve error messages for trailing comma on from import (GH-20294)
72e0aa2

New changeset 275d7e1 by Pablo Galindo in branch '3.9':
[3.9] bpo-40176: Improve error messages for trailing comma on from import (GH-20294) (GH-20302)
275d7e1

New changeset a698d52 by Batuhan Taskaya in branch 'master':
bpo-40176: Improve error messages for unclosed string literals (GH-19346)
a698d52