5 files changed, 26 insertions, 0 deletions
diff --git a/Include/errcode.h b/Include/errcode.h
index f2671d6..2e07fc2 100644
--- a/Include/errcode.h
+++ b/Include/errcode.h
@@ -28,6 +28,7 @@ extern "C" {
 #define E_DECODE        22      /* Error in decoding into Unicode */
 #define E_LINECONT      25      /* Unexpected characters after a line continuation */
 #define E_BADSINGLE     27      /* Ill-formed single statement input */
+#define E_INTERACT_STOP 28      /* Interactive mode stopped tokenization */
 
 #ifdef __cplusplus
 }
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-05-21-21-16-03.bpo-44201.bGaSjt.rst b/Misc/NEWS.d/next/Core and Builtins/2021-05-21-21-16-03.bpo-44201.bGaSjt.rst
new file mode 100644
index 0000000..6f61aac
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-05-21-21-16-03.bpo-44201.bGaSjt.rst
@@ -0,0 +1,3 @@
+Avoid side effects of checking for specialized syntax errors in the REPL
+that was causing it to ask for extra tokens after a syntax error had been
+detected. Patch by Pablo Galindo
diff --git a/Parser/pegen.c b/Parser/pegen.c
index 3c25e4d..548a647 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -1234,6 +1234,9 @@ reset_parser_state(Parser *p)
     }
     p->mark = 0;
     p->call_invalid_rules = 1;
+    // Don't try to get extra tokens in interactive mode when trying to
+    // raise specialized errors in the second pass.
+    p->tok->interactive_underflow = IUNDERFLOW_STOP;
 }
 
 static int
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index ad32293..a86af9b 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -85,6 +85,7 @@ tok_new(void)
     tok->async_def = 0;
     tok->async_def_indent = 0;
     tok->async_def_nl = 0;
+    tok->interactive_underflow = IUNDERFLOW_NORMAL;
 
     return tok;
 }
@@ -845,6 +846,10 @@ tok_underflow_string(struct tok_state *tok) {
 
 static int
 tok_underflow_interactive(struct tok_state *tok) {
+    if (tok->interactive_underflow == IUNDERFLOW_STOP) {
+        tok->done = E_INTERACT_STOP;
+        return 1;
+    }
     char *newtok = PyOS_Readline(stdin, stdout, tok->prompt);
     if (newtok != NULL) {
         char *translated = translate_newlines(newtok, 0, tok);
@@ -1399,6 +1404,10 @@ tok_get(struct tok_state *tok, const char **p_start, const char **p_end)
         }
     }
 
+    if (tok->done == E_INTERACT_STOP) {
+        return ENDMARKER;
+    }
+
     /* Check for EOF and errors now */
     if (c == EOF) {
         if (tok->level) {
diff --git a/Parser/tokenizer.h b/Parser/tokenizer.h
index aaa31f3..ff563d5 100644
--- a/Parser/tokenizer.h
+++ b/Parser/tokenizer.h
@@ -19,6 +19,14 @@ enum decoding_state {
     STATE_NORMAL
 };
 
+enum interactive_underflow_t {
+    /* Normal mode of operation: return a new token when asked in interactie mode */
+    IUNDERFLOW_NORMAL,
+    /* Forcefully return ENDMARKER when asked for a new token in interactive mode. This
+     * can be used to prevent the tokenizer to promt the user for new tokens */
+    IUNDERFLOW_STOP,
+};
+
 /* Tokenizer state */
 struct tok_state {
     /* Input state; buf <= cur <= inp <= end */
@@ -74,6 +82,8 @@ struct tok_state {
     int async_def_indent; /* Indentation level of the outermost 'async def'. */
     int async_def_nl;     /* =1 if the outermost 'async def' had at least one
                              NEWLINE token after it. */
+    /* How to proceed when asked for a new token in interactive mode */
+    enum interactive_underflow_t interactive_underflow; 
 };
 
 extern struct tok_state *PyTokenizer_FromString(const char *, int);