fix problem with const char* value, that return PyUnicode_AsUTF8AndSize in Python3.7 case; added docstring to get_string_data func

Author: anmyachev

pandas/_libs/tslibs/np_datetime.pyx

@@ -33,7 +33,7 @@ cdef extern from "src/datetime/np_datetime.h":
     npy_datetimestruct _NS_MIN_DTS, _NS_MAX_DTS
 
 cdef extern from "src/datetime/np_datetime_strings.h":
-    int parse_iso_8601_datetime(char *str, int len,
+    int parse_iso_8601_datetime(const char *str, int len,
                                 npy_datetimestruct *out,
                                 int *out_local, int *out_tzoffset)
 
@@ -175,7 +175,7 @@ cdef inline int _string_to_dts(object val, npy_datetimestruct* dts,
                                int* out_local, int* out_tzoffset) except? -1:
     cdef:
         Py_ssize_t length
-        char *tmp
+        const char* tmp
 
     if not get_string_data(val, &tmp, &length):
         raise ValueError('Unable to parse %s' % str(val))

pandas/_libs/tslibs/src/datetime/np_datetime_strings.c

@@ -66,12 +66,13 @@ This file implements string parsing and creation for NumPy datetime.
  *
  * Returns 0 on success, -1 on failure.
  */
-int parse_iso_8601_datetime(char *str, int len,
+int parse_iso_8601_datetime(const char *str, int len,
                             npy_datetimestruct *out,
                             int *out_local, int *out_tzoffset) {
     int year_leap = 0;
     int i, numdigits;
-    char *substr, sublen;
+    const char *substr;
+    char sublen;
 
     /* If year-month-day are separated by a valid separator,
      * months/days without leading zeroes will be parsed

pandas/_libs/tslibs/src/datetime/np_datetime_strings.h

@@ -54,7 +54,7 @@ This file implements string parsing and creation for NumPy datetime.
  * Returns 0 on success, -1 on failure.
  */
 int
-parse_iso_8601_datetime(char *str, int len,
+parse_iso_8601_datetime(const char *str, int len,
                         npy_datetimestruct *out,
                         int *out_local,
                         int *out_tzoffset);

pandas/_libs/tslibs/util.pxd

@@ -232,10 +232,27 @@ cdef inline bint is_nan(object val):
     return (is_float_object(val) or is_complex_object(val)) and val != val
 
 
-cdef inline bint get_string_data(object s, char **buf, Py_ssize_t *length):
+cdef inline bint get_string_data(object s, const char **buf,
+                                 Py_ssize_t *length):
+    """
+    Extract internal char * buffer of unicode or bytes object `s` to `buf` with
+    getting length of this internal buffer, that save in `length`.
+    Return `False` if it failed to extract such buffer for whatever reason
+    otherwise return `True`
+
+    Parameters
+    ----------
+    s      : object
+    buf    : const char**
+    length : Py_ssize_t*
+    
+    Returns
+    -------
+    bint
+    """
     if PyUnicode_Check(s):
         buf[0] = PyUnicode_AsUTF8AndSize(s, length)
         return buf[0] != NULL
     if PyBytes_Check(s):
-        return PyBytes_AsStringAndSize(s, buf, length) == 0
+        return PyBytes_AsStringAndSize(s, <char**>buf, length) == 0
     return False