[FIXED] Why in Python 3 do quadruple quotes produce a Syntax error?

Issue

I can add additional quotes to the beginning of a triple-quoted string, but not to the end. Why is that? This block of code:

print(""""
String that starts with quadruple quotes and ends with triple quotes
""")

Produces this output:

"
String that starts with quadruple quotes and ends with triple quotes

Yet this code block doesn't work:

print(""""
String that starts with quadruple quotes and ends with quadruple quotes
"""")

It produces this error:

  File "example.py", line 3
    """")
        ^
SyntaxError: EOL while scanning string literal

I don't ever need to use a quadruple-quote string, but I'm curious why Python won't let me do it. Can anyone help me understand?

Solution

You can't use """ anywhere in the value of a triple-quoted string. Not at the start, and not at the end.

That's because, after the first three """ opening characters denoting the start of such a string, another sequence of """ is always going to be the end of the string. Your fourth " lies outside of the string object you created, and a single " without a closing " is not a valid string.

Python has no other method of knowing when such a string ends. You can't arbitrarily extend the string 'inwards' with additional " characters before the final """, because that'd be indistinguishable from the valid and legal^*:

>>> """string 1"""" string 2"
'string 1 string 2'

If you must include a " before the closing """, escape it. You can do so by preceding it with a backslash:

>>> """This is triple-quoted string that
... ends in a single double quote: \""""
'This is triple-quoted string that\nends in a single double quote: "'

Note that there is no such thing as a quadruple-quote string. Python doesn't let you combine " quotes into longer sequences arbitrarily. Only "single quoted" and """triple-quoted""" syntax exists (using " or '). The rules for a triple-quoted string differ from a single-quoted string; newlines are allowed in the former, not in the latter.

See the String and Bytes literals section of the reference documentation for more details, which defines the grammar as:

shortstring     ::=  "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring      ::=  "'''" longstringitem* "'''" | '"""' longstringitem* '"""'

and explicitly mentions:

In triple-quoted literals, unescaped newlines and quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the literal. (A “quote” is the character used to open the literal, i.e. either ' or ".)

(bold emphasis mine).

---

^* The expression is legal because it consists of two string literals, one with """ quoting, the next with " quoting. Consecutive string literals are automatically concatenated, just like they would in C. See String literal concatenation.

Answered By - Martijn Pieters