Parameterize NA sentinel for hashtables

[TomAugspurger]

xref #19938. There's I'm abusing the fact that the Int64HashTable class treats iNaT as a missing value. It'd be cleaner to just pass the Categorical's codes with a flag saying that -1 is the missing value marker.

IIUC, the missing value condition is baked into the class definition.

{{py:

# name, dtype, null_condition, float_group
dtypes = [('Float64', 'float64', 'val != val', True),
          ('UInt64', 'uint64', 'False', False),
          ('Int64', 'int64', 'val == iNaT', False)]

I'm not familiar enough with Cython to know how that could be made a parameter. Possibly an optional scalar value. If that value is not None check if val == na_sentinel. Else, use the condition from the class definition?

[WillAyd]

Not 100% certain but I believe you can define an optional argument for your sentinel in the __cinit__

pandas/pandas/_libs/hashtable_class_helper.pxi.in

Line 311 in 3783ccc

def __cinit__(self, size_hint=1):

Then adjust the existing comparison to do something like if check_null and (val == self.sentinel or {{null_condition}}):

pandas/pandas/_libs/hashtable_class_helper.pxi.in

Line 428 in 3783ccc

if check_null and {{null_condition}}:

[jreback]

yeah maybe we could just make a parameter for this, rather than check_null, and default it in the template (so allow it to be overriden)

[TomAugspurger]

Just doing if val == self.na_value doesn't quite work, since na_value is a (potentially?) a Python object, and this is all in a no_gil block.

[WillAyd]

I don't think an object is possible to hit that branch because it's not explicitly listed as one of the dtypes - do you have an example of where that is happening?

pandas/pandas/_libs/hashtable_class_helper.pxi.in

Line 254 in 3783ccc

dtypes = [('Float64', 'float64', 'val != val', True),

[WillAyd]

If you need to add support for objects in your template you could replace the nogil block with if True: - I've done this a few times in groupby_helper.pxi.in:

pandas/pandas/_libs/groupby_helper.pxi.in

Line 329 in 3783ccc

if True: # make templating happy

[TomAugspurger]

I don't think an object is possible to hit that branch because it's not explicitly listed as one of the dtypes

It'd be a parameter passed by the user when the hashtable is created.

I'm probably just not familiar enough with cython to describe what's going on. I want something roughly like this

diff --git a/pandas/_libs/hashtable_class_helper.pxi.in b/pandas/_libs/hashtable_class_helper.pxi.in
index bca4e388f..58df8f8b7 100644
--- a/pandas/_libs/hashtable_class_helper.pxi.in
+++ b/pandas/_libs/hashtable_class_helper.pxi.in
@@ -308,8 +308,9 @@ def get_dispatch(dtypes):
 
 cdef class {{name}}HashTable(HashTable):
 
-    def __cinit__(self, size_hint=1):
+    def __cinit__(self, size_hint=1, {{dtype}}_t na_value=None):
         self.table = kh_init_{{dtype}}()
+        self.na_value = na_value
         if size_hint is not None:
             kh_resize_{{dtype}}(self.table, size_hint)
 
@@ -414,18 +415,25 @@ cdef class {{name}}HashTable(HashTable):
             int64_t[:] labels
             Py_ssize_t idx, count = count_prior
             int ret = 0
-            {{dtype}}_t val
+            {{dtype}}_t val, na_value
             khiter_t k
             {{name}}VectorData *ud
+            bint use_na_value
 
         labels = np.empty(n, dtype=np.int64)
         ud = uniques.data
+        na_value = self.na_value
+        use_na_value = self.na_value != None
 
         with nogil:
             for i in range(n):
                 val = values[i]
 
-                if check_null and {{null_condition}}:
+                if use_na_value and val == na_value:
+                    labels[i] = na_sentinel
+                    continue
+
+                elif check_null and {{null_condition}}:
                     labels[i] = na_sentinel
                     continue
 
diff --git a/pandas/core/algorithms.py b/pandas/core/algorithms.py
index de2e63826..825d17b2c 100644
--- a/pandas/core/algorithms.py
+++ b/pandas/core/algorithms.py
@@ -435,7 +435,7 @@ def isin(comps, values):
     return f(comps, values)
 
 
-def _factorize_array(values, check_nulls, na_sentinel=-1, size_hint=None):
+def _factorize_array(values, check_nulls, na_sentinel=-1, size_hint=None, na_value=None):
     """Factorize an array-like to labels and uniques.
 
     This doesn't do any coercion of types or unboxing before factorization.
@@ -455,7 +455,7 @@ def _factorize_array(values, check_nulls, na_sentinel=-1, size_hint=None):
     """
     (hash_klass, vec_klass), values = _get_data_algo(values, _hashtables)
 
-    table = hash_klass(size_hint or len(values))
+    table = hash_klass(size_hint or len(values), na_value=na_value)
     uniques = vec_klass()
     labels = table.get_labels(values, uniques, 0, na_sentinel, check_nulls)
 
@@ -465,7 +465,7 @@ def _factorize_array(values, check_nulls, na_sentinel=-1, size_hint=None):
 
 
 @deprecate_kwarg(old_arg_name='order', new_arg_name=None)
-def factorize(values, sort=False, order=None, na_sentinel=-1, size_hint=None):
+def factorize(values, sort=False, order=None, na_sentinel=-1, size_hint=None, na_value=None):
     """
     Encode input values as an enumerated type or categorical variable
 
@@ -511,7 +511,7 @@ def factorize(values, sort=False, order=None, na_sentinel=-1, size_hint=None):
         check_nulls = not is_integer_dtype(original)
         labels, uniques = _factorize_array(values, check_nulls,
                                            na_sentinel=na_sentinel,
-                                           size_hint=size_hint)
+                                           size_hint=size_hint, na_value=na_value)
 
     if sort and len(uniques) > 0:
         from pandas.core.sorting import safe_sort

This compiles, but doesn't run as float64_t na_value=None isn't valid, since None isn't a float. Is there a way around that with Cython? Otherwise, we'll need type-specific NA values (NaN, iNaT, something). But then we won't have a way to disable that.

Unless... at the python level, the default is None, we check if the user specifies it, and pass through an additional flag telling the hashtable, use_na_value. That should work