[Backport pandas-dev#14514] BUG: Don't parse inline quotes in skipped lines (pandas-dev#14514)

Closes pandas-devgh-14459.
(cherry picked from commit b088112)

Author: gfyoung

doc/source/whatsnew/v0.19.1.txt

@@ -36,6 +36,7 @@ Bug Fixes
 - Compat with Cython 0.25 for building (:issue:`14496`)
 
 
+- Bug in ``pd.read_csv`` for the C engine in which quotation marks were improperly parsed in skipped rows (:issue:`14459`)
 - Bug in ``pd.read_csv`` for Python 2.x in which Unicode quote characters were no longer being respected (:issue:`14477`)
 - Bug in localizing an ambiguous timezone when a boolean is passed (:issue:`14402`)
 - Bug in ``TimedeltaIndex`` addition with a Datetime-like object where addition overflow in the negative direction was not being caught (:issue:`14068`, :issue:`14453`)

pandas/io/tests/parser/skiprows.py

@@ -190,3 +190,11 @@ def test_skiprows_lineterminator(self):
                                skiprows=1, delim_whitespace=True,
                                names=['date', 'time', 'var', 'flag', 'oflag'])
             tm.assert_frame_equal(df, expected)
+
+    def test_skiprows_infield_quote(self):
+        # see gh-14459
+        data = 'a"\nb"\na\n1'
+        expected = DataFrame({'a': [1]})
+
+        df = self.read_csv(StringIO(data), skiprows=2)
+        tm.assert_frame_equal(df, expected)

pandas/src/parser/tokenizer.c

@@ -478,9 +478,10 @@ static int end_line(parser_t *self) {
         }
     }
 
-    if (self->state == SKIP_LINE || \
-        self->state == QUOTE_IN_SKIP_LINE || \
-        self->state == QUOTE_IN_QUOTE_IN_SKIP_LINE
+    if (self->state == START_FIELD_IN_SKIP_LINE || \
+        self->state == IN_FIELD_IN_SKIP_LINE || \
+        self->state == IN_QUOTED_FIELD_IN_SKIP_LINE || \
+        self->state == QUOTE_IN_QUOTED_FIELD_IN_SKIP_LINE
     ) {
         TRACE(("end_line: Skipping row %d\n", self->file_lines));
         // increment file line count
@@ -761,38 +762,54 @@ int tokenize_bytes(parser_t *self, size_t line_limit)
 
         switch(self->state) {
 
-        case SKIP_LINE:
-            TRACE(("tokenize_bytes SKIP_LINE 0x%x, state %d\n", c, self->state));
+        case START_FIELD_IN_SKIP_LINE:
             if (IS_TERMINATOR(c)) {
                 END_LINE();
             } else if (IS_CARRIAGE(c)) {
                 self->file_lines++;
                 self->state = EAT_CRNL_NOP;
             } else if (IS_QUOTE(c)) {
-                self->state = QUOTE_IN_SKIP_LINE;
+                self->state = IN_QUOTED_FIELD_IN_SKIP_LINE;
+            } else if (IS_DELIMITER(c)) {
+                // Do nothing, we're starting a new field again.
+            } else {
+                self->state = IN_FIELD_IN_SKIP_LINE;
+            }
+            break;
+
+        case IN_FIELD_IN_SKIP_LINE:
+            if (IS_TERMINATOR(c)) {
+                END_LINE();
+            } else if (IS_CARRIAGE(c)) {
+                self->file_lines++;
+                self->state = EAT_CRNL_NOP;
+            } else if (IS_DELIMITER(c)) {
+                self->state = START_FIELD_IN_SKIP_LINE;
             }
             break;
 
-        case QUOTE_IN_SKIP_LINE:
+        case IN_QUOTED_FIELD_IN_SKIP_LINE:
             if (IS_QUOTE(c)) {
                 if (self->doublequote) {
-                    self->state = QUOTE_IN_QUOTE_IN_SKIP_LINE;
+                    self->state = QUOTE_IN_QUOTED_FIELD_IN_SKIP_LINE;
                 } else {
-                    self->state = SKIP_LINE;
+                    self->state = IN_FIELD_IN_SKIP_LINE;
                 }
             }
             break;
 
-        case QUOTE_IN_QUOTE_IN_SKIP_LINE:
+        case QUOTE_IN_QUOTED_FIELD_IN_SKIP_LINE:
             if (IS_QUOTE(c)) {
-                self->state = QUOTE_IN_SKIP_LINE;
+                self->state = IN_QUOTED_FIELD_IN_SKIP_LINE;
             } else if (IS_TERMINATOR(c)) {
                 END_LINE();
             } else if (IS_CARRIAGE(c)) {
                 self->file_lines++;
                 self->state = EAT_CRNL_NOP;
+            } else if (IS_DELIMITER(c)) {
+                self->state = START_FIELD_IN_SKIP_LINE;
             } else {
-                self->state = SKIP_LINE;
+                self->state = IN_FIELD_IN_SKIP_LINE;
             }
             break;
 
@@ -846,9 +863,9 @@ int tokenize_bytes(parser_t *self, size_t line_limit)
             // start of record
             if (skip_this_line(self, self->file_lines)) {
                 if (IS_QUOTE(c)) {
-                    self->state = QUOTE_IN_SKIP_LINE;
+                    self->state = IN_QUOTED_FIELD_IN_SKIP_LINE;
                 } else {
-                    self->state = SKIP_LINE;
+                    self->state = IN_FIELD_IN_SKIP_LINE;
 
                     if (IS_TERMINATOR(c)) {
                         END_LINE();

pandas/src/parser/tokenizer.h

@@ -123,9 +123,10 @@ typedef enum {
     EAT_COMMENT,
     EAT_LINE_COMMENT,
     WHITESPACE_LINE,
-    SKIP_LINE,
-    QUOTE_IN_SKIP_LINE,
-    QUOTE_IN_QUOTE_IN_SKIP_LINE,
+    START_FIELD_IN_SKIP_LINE,
+    IN_FIELD_IN_SKIP_LINE,
+    IN_QUOTED_FIELD_IN_SKIP_LINE,
+    QUOTE_IN_QUOTED_FIELD_IN_SKIP_LINE,
     FINISHED
 } ParserState;