pythongh-134675: Add t-string prefixes to tokenizer module, lexical analysis doc, and add a test to make sure we catch this error in the future. (python#134734)

* Add t-string prefixes to _all_string_prefixes, and add a test to make sure we catch this error in the future.

* Update lexical analysis docs for t-string prefixes.

Author: ericvsmith

Doc/reference/lexical_analysis.rst

@@ -489,8 +489,9 @@ String literals are described by the following lexical definitions:
 
 .. productionlist:: python-grammar
    stringliteral: [`stringprefix`](`shortstring` | `longstring`)
-   stringprefix: "r" | "u" | "R" | "U" | "f" | "F"
+   stringprefix: "r" | "u" | "R" | "U" | "f" | "F" | "t" | "T"
                : | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
+               : | "tr" | "Tr" | "tR" | "TR" | "rt" | "rT" | "Rt" | "RT"
    shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"'
    longstring: "'''" `longstringitem`* "'''" | '"""' `longstringitem`* '"""'
    shortstringitem: `shortstringchar` | `stringescapeseq`

Lib/test/test_tokenize.py

@@ -1,6 +1,8 @@
 import contextlib
+import itertools
 import os
 import re
+import string
 import tempfile
 import token
 import tokenize
@@ -3238,5 +3240,59 @@ def test_exact_flag(self):
             self.check_output(source, expect, flag)
 
 
+class StringPrefixTest(unittest.TestCase):
+    def test_prefixes(self):
+        # Get the list of defined string prefixes.  I don't see an
+        # obvious documented way of doing this, but probably the best
+        # thing is to split apart tokenize.StringPrefix.
+
+        # Make sure StringPrefix begins and ends in parens.
+        self.assertEqual(tokenize.StringPrefix[0], '(')
+        self.assertEqual(tokenize.StringPrefix[-1], ')')
+
+        # Then split apart everything else by '|'.
+        defined_prefixes = set(tokenize.StringPrefix[1:-1].split('|'))
+
+        # Now compute the actual string prefixes, by exec-ing all
+        # valid prefix combinations, followed by an empty string.
+
+        # Try all prefix lengths until we find a length that has zero
+        # valid prefixes.  This will miss the case where for example
+        # there are no valid 3 character prefixes, but there are valid
+        # 4 character prefixes.  That seems extremely unlikely.
+
+        # Note that the empty prefix is being included, because length
+        # starts at 0.  That's expected, since StringPrefix includes
+        # the empty prefix.
+
+        valid_prefixes = set()
+        for length in itertools.count():
+            num_at_this_length = 0
+            for prefix in (
+                "".join(l) for l in list(itertools.combinations(string.ascii_lowercase, length))
+            ):
+                for t in itertools.permutations(prefix):
+                    for u in itertools.product(*[(c, c.upper()) for c in t]):
+                        p = ''.join(u)
+                        if p == "not":
+                            # 'not' can never be a string prefix,
+                            # because it's a valid expression: not ""
+                            continue
+                        try:
+                            eval(f'{p}""')
+
+                            # No syntax error, so p is a valid string
+                            # prefix.
+
+                            valid_prefixes.add(p)
+                            num_at_this_length += 1
+                        except SyntaxError:
+                            pass
+            if num_at_this_length == 0:
+                break
+
+        self.assertEqual(defined_prefixes, valid_prefixes)
+
+
 if __name__ == "__main__":
     unittest.main()

Lib/tokenize.py

@@ -86,7 +86,7 @@ def _all_string_prefixes():
     # The valid string prefixes. Only contain the lower case versions,
     #  and don't contain any permutations (include 'fr', but not
     #  'rf'). The various permutations will be generated.
-    _valid_string_prefixes = ['b', 'r', 'u', 'f', 'br', 'fr']
+    _valid_string_prefixes = ['b', 'r', 'u', 'f', 't', 'br', 'fr', 'tr']
     # if we add binary f-strings, add: ['fb', 'fbr']
     result = {''}
     for prefix in _valid_string_prefixes: