PERF: cythonizing _concat_date_cols; conversion to float without exceptions in _does_string_look_like_datetime

asv_bench/benchmarks/io/csv.py

@@ -96,6 +96,35 @@ def time_read_csv(self, infer_datetime_format, format):
                  infer_datetime_format=infer_datetime_format)
 
 
+class ReadCSVConcatDatetime(StringIORewind):
+
+    iso8601 = '%Y-%m-%d %H:%M:%S'
+
+    def setup(self):
+        rng = date_range('1/1/2000', periods=50000, freq='S')
+        self.StringIO_input = StringIO('\n'.join(
+                                       rng.strftime(self.iso8601).tolist()))
+
+    def time_read_csv(self):
+        read_csv(self.data(self.StringIO_input),
+                 header=None, names=['foo'], parse_dates=['foo'],
+                 infer_datetime_format=False)
+
+
+class ReadCSVConcatDatetimeBadDateValue(StringIORewind):
+
+    params = (['nan', '0', ''],)
+    param_names = ['bad_date_value']
+
+    def setup(self, bad_date_value):
+        self.StringIO_input = StringIO(('%s,\n' % bad_date_value) * 50000)
+
+    def time_read_csv(self, bad_date_value):
+        read_csv(self.data(self.StringIO_input),
+                 header=None, names=['foo', 'bar'], parse_dates=['foo'],
+                 infer_datetime_format=False)
+
+
 class ReadCSVSkipRows(BaseIO):
 
     fname = '__test__.csv'

asv_bench/benchmarks/io/parsers.py

@@ -0,0 +1,34 @@
+import numpy as np
+
+from pandas._libs.tslibs.parsing import (
+    _concat_date_cols, _does_string_look_like_datetime)
+
+
+class DoesStringLookLikeDatetime(object):
+
+    params = (['2Q2005', '0.0', '10000'],)
+    param_names = ['value']
+
+    def setup(self, value):
+        self.objects = [value] * 1000000
+
+    def time_check_datetimes(self, value):
+        for obj in self.objects:
+            _does_string_look_like_datetime(obj)
+
+
+class ConcatDateCols(object):
+
+    params = ([1234567890, 'AAAA'], [1, 2])
+    param_names = ['value', 'dim']
+
+    def setup(self, value, dim):
+        count_elem = 10000
+        if dim == 1:
+            self.object = (np.array([value] * count_elem),)
+        if dim == 2:
+            self.object = (np.array([value] * count_elem),
+                           np.array([value] * count_elem))
+
+    def time_check_concat(self, value, dim):
+        _concat_date_cols(self.object)

doc/source/whatsnew/v0.25.0.rst

@@ -252,6 +252,8 @@ Performance Improvements
 - Improved performance of :meth:`read_csv` by much faster parsing of ``MM/YYYY`` and ``DD/MM/YYYY`` datetime formats (:issue:`25922`)
 - Improved performance of nanops for dtypes that cannot store NaNs. Speedup is particularly prominent for :meth:`Series.all` and :meth:`Series.any` (:issue:`25070`)
 - Improved performance of :meth:`Series.map` for dictionary mappers on categorical series by mapping the categories instead of mapping all values (:issue:`23785`)
+- Improved performance of :meth:`read_csv` by faster concatenating date columns without extra conversion to string for integer/float zero
+  and float NaN; by faster checking the string for the possibility of being a date (:issue:`25754`)
 
 .. _whatsnew_0250.bug_fixes:
 

pandas/_libs/lib.pyx

@@ -8,10 +8,9 @@ import warnings
 import cython
 from cython import Py_ssize_t
 
-from cpython cimport (Py_INCREF, PyTuple_SET_ITEM,
-                      PyTuple_New,
-                      Py_EQ,
-                      PyObject_RichCompareBool)
+from cpython cimport (Py_INCREF, PyTuple_SET_ITEM, PyTuple_New, PyObject_Str,
+                      Py_EQ, Py_SIZE, PyObject_RichCompareBool,
+                      PyUnicode_Join, PyList_New)
 
 from cpython.datetime cimport (PyDateTime_Check, PyDate_Check,
                                PyTime_Check, PyDelta_Check,
@@ -23,10 +22,8 @@ cimport numpy as cnp
 from numpy cimport (ndarray, PyArray_GETITEM,
                     PyArray_ITER_DATA, PyArray_ITER_NEXT, PyArray_IterNew,
                     flatiter, NPY_OBJECT,
-                    int64_t,
-                    float32_t, float64_t,
-                    uint8_t, uint64_t,
-                    complex128_t)
+                    int64_t, float32_t, float64_t,
+                    uint8_t, uint64_t, complex128_t)
 cnp.import_array()
 
 cdef extern from "numpy/arrayobject.h":

pandas/_libs/tslibs/parsing.pyx

@@ -7,11 +7,19 @@ from io import StringIO
 
 from libc.string cimport strchr
 
+import cython
+
+from cpython cimport PyObject_Str, PyUnicode_Join
+
 from cpython.datetime cimport datetime, datetime_new, import_datetime
 from cpython.version cimport PY_VERSION_HEX
 import_datetime()
 
 import numpy as np
+cimport numpy as cnp
+from numpy cimport (PyArray_GETITEM, PyArray_ITER_DATA, PyArray_ITER_NEXT,
+                    PyArray_IterNew, flatiter, float64_t)
+cnp.import_array()
 
 # dateutil compat
 from dateutil.tz import (tzoffset,
@@ -26,11 +34,16 @@ from pandas._config import get_option
 
 from pandas._libs.tslibs.ccalendar import MONTH_NUMBERS
 from pandas._libs.tslibs.nattype import nat_strings, NaT
-from pandas._libs.tslibs.util cimport get_c_string_buf_and_size
+from pandas._libs.tslibs.util cimport is_array, get_c_string_buf_and_size
 
 cdef extern from "../src/headers/portable.h":
     int getdigit_ascii(char c, int default) nogil
 
+cdef extern from "../src/parser/tokenizer.h":
+    double xstrtod(const char *p, char **q, char decimal, char sci, char tsep,
+                   int skip_trailing, int *error, int *maybe_int)
+
+
 # ----------------------------------------------------------------------
 # Constants
 
@@ -302,20 +315,48 @@ cdef parse_datetime_string_with_reso(date_string, freq=None, dayfirst=False,
     return parsed, parsed, reso
 
 
-cpdef bint _does_string_look_like_datetime(object date_string):
-    if date_string.startswith('0'):
-        # Strings starting with 0 are more consistent with a
-        # date-like string than a number
-        return True
+cpdef bint _does_string_look_like_datetime(object py_string):
+    """
+    Checks whether given string is a datetime: it has to start with '0' or
+    be greater than 1000.
 
-    try:
-        if float(date_string) < 1000:
-            return False
-    except ValueError:
-        pass
+    Parameters
+    ----------
+    py_string: object
 
-    if date_string in _not_datelike_strings:
-        return False
+    Returns
+    -------
+    whether given string is a datetime
+    """
+    cdef:
+        const char *buf
+        char *endptr = NULL
+        Py_ssize_t length = -1
+        double converted_date
+        char first
+        int error = 0
+
+    buf = get_c_string_buf_and_size(py_string, &length)
+    if length >= 1:
+        first = buf[0]
+        if first == b'0':
+            # Strings starting with 0 are more consistent with a
+            # date-like string than a number
+            return True
+        elif py_string in _not_datelike_strings:
+            return False
+        else:
+            # xstrtod with such paramaters copies behavior of python `float`
+            # cast; for example, " 35.e-1 " is valid string for this cast so,
+            # for correctly xstrtod call necessary to pass these params:
+            # b'.' - a dot is used as separator, b'e' - an exponential form of
+            # a float number can be used, b'\0' - not to use a thousand
+            # separator, 1 - skip extra spaces before and after,
+            converted_date = xstrtod(buf, &endptr,
+                                     b'.', b'e', b'\0', 1, &error, NULL)
+            # if there were no errors and the whole line was parsed, then ...
+            if error == 0 and endptr == buf + length:
+                return converted_date >= 1000
 
     return True
 
@@ -857,3 +898,117 @@ def _guess_datetime_format(dt_str, dayfirst=False, dt_str_parse=du_parse,
         return guessed_format
     else:
         return None
+
+
+@cython.wraparound(False)
+@cython.boundscheck(False)
+cdef inline object convert_to_unicode(object item,
+                                      bint keep_trivial_numbers):
+    """
+    Convert `item` to str.
+
+    Parameters
+    ----------
+    item : object
+    keep_trivial_numbers : bool
+        if True, then conversion (to string from integer/float zero)
+        is not performed
+
+    Returns
+    -------
+    str or int or float
+    """
+    cdef:
+        float64_t float_item
+
+    if keep_trivial_numbers:
+        if isinstance(item, int):
+            if <int>item == 0:
+                return item
+        elif isinstance(item, float):
+            float_item = item
+            if float_item == 0.0 or float_item != float_item:
+                return item
+
+    if not isinstance(item, str):
+        item = PyObject_Str(item)
+
+    return item
+
+
+@cython.wraparound(False)
+@cython.boundscheck(False)
+def _concat_date_cols(tuple date_cols, bint keep_trivial_numbers=True):
+    """
+    Concatenates elements from numpy arrays in `date_cols` into strings.
+
+    Parameters
+    ----------
+    date_cols : tuple of numpy arrays
+    keep_trivial_numbers : bool, default True
+        if True and len(date_cols) == 1, then
+        conversion (to string from integer/float zero) is not performed
+
+    Returns
+    -------
+    arr_of_rows : ndarray (dtype=object)
+
+    Examples
+    --------
+    >>> dates=np.array(['3/31/2019', '4/31/2019'], dtype=object)
+    >>> times=np.array(['11:20', '10:45'], dtype=object)
+    >>> result = _concat_date_cols((dates, times))
+    >>> result
+    array(['3/31/2019 11:20', '4/31/2019 10:45'], dtype=object)
+    """
+    cdef:
+        Py_ssize_t rows_count = 0, col_count = len(date_cols)
+        Py_ssize_t col_idx, row_idx
+        list list_to_join
+        cnp.ndarray[object] iters
+        object[::1] iters_view
+        flatiter it
+        cnp.ndarray[object] result
+        object[:] result_view
+
+    if col_count == 0:
+        return np.zeros(0, dtype=object)
+
+    if not all(is_array(array) for array in date_cols):
+        raise ValueError("not all elements from date_cols are numpy arrays")
+
+    rows_count = min(len(array) for array in date_cols)
+    result = np.zeros(rows_count, dtype=object)
+    result_view = result
+
+    if col_count == 1:
+        array = date_cols[0]
+        it = <flatiter>PyArray_IterNew(array)
+        for row_idx in range(rows_count):
+            item = PyArray_GETITEM(array, PyArray_ITER_DATA(it))
+            result_view[row_idx] = convert_to_unicode(item,
+                                                      keep_trivial_numbers)
+            PyArray_ITER_NEXT(it)
+    else:
+        # create fixed size list - more effecient memory allocation
+        list_to_join = [None] * col_count
+        iters = np.zeros(col_count, dtype=object)
+
+        # create memoryview of iters ndarray, that will contain some
+        # flatiter's for each array in `date_cols` - more effecient indexing
+        iters_view = iters
+        for col_idx, array in enumerate(date_cols):
+            iters_view[col_idx] = PyArray_IterNew(array)
+
+        # array elements that are on the same line are converted to one string
+        for row_idx in range(rows_count):
+            for col_idx, array in enumerate(date_cols):
+                # this cast is needed, because we did not find a way
+                # to efficiently store `flatiter` type objects in ndarray
+                it = <flatiter>iters_view[col_idx]
+                item = PyArray_GETITEM(array, PyArray_ITER_DATA(it))
+                list_to_join[col_idx] = convert_to_unicode(item, False)
+                PyArray_ITER_NEXT(it)
+            result_view[row_idx] = PyUnicode_Join(' ', list_to_join)
+
+    return result

pandas/io/parsers.py

@@ -3186,7 +3186,7 @@ def _make_date_converter(date_parser=None, dayfirst=False,
                          infer_datetime_format=False, cache_dates=True):
     def converter(*date_cols):
         if date_parser is None:
-            strs = _concat_date_cols(date_cols)
+            strs = parsing._concat_date_cols(date_cols)
 
             try:
                 return tools.to_datetime(
@@ -3216,10 +3216,10 @@ def converter(*date_cols):
             except Exception:
                 try:
                     return tools.to_datetime(
-                        parsing.try_parse_dates(_concat_date_cols(date_cols),
-                                                parser=date_parser,
-                                                dayfirst=dayfirst),
-                        cache=cache_dates,
+                        parsing.try_parse_dates(
+                            parsing._concat_date_cols(date_cols),
+                            parser=date_parser,
+                            dayfirst=dayfirst),
                         errors='ignore')
                 except Exception:
                     return generic_parser(date_parser, *date_cols)
@@ -3511,15 +3511,6 @@ def _get_col_names(colspec, columns):
     return colnames
 
 
-def _concat_date_cols(date_cols):
-    if len(date_cols) == 1:
-        return np.array([str(x) for x in date_cols[0]], dtype=object)
-
-    rs = np.array([' '.join(str(y) for y in x)
-                   for x in zip(*date_cols)], dtype=object)
-    return rs
-
-
 class FixedWidthReader(BaseIterator):
     """
     A reader of fixed-width lines.