API: to_datetime, required unit with numerical (pandas-dev#15836)

* Tests
  * add test_to_datetime_numerical_input
  * add fixtures
  * update bool tests
* Check argument
  * for numerical type
  * DataFrame with unit

Author: atbd

pandas/_libs/tslib.pyx

@@ -2308,7 +2308,7 @@ cpdef array_with_unit_to_datetime(ndarray values, unit, errors='coerce'):
             else:
 
                 if is_raise:
-                    raise ValueError("non convertible value {0}"
+                    raise ValueError("non convertible value {0} "
                                      "with the unit '{1}'".format(
                                          val,
                                          unit))

pandas/core/tools/datetimes.py

@@ -14,7 +14,8 @@
     is_float,
     is_list_like,
     is_scalar,
-    is_numeric_dtype)
+    is_numeric_dtype,
+    is_bool_dtype)
 from pandas.core.dtypes.generic import (
     ABCIndexClass, ABCSeries,
     ABCDataFrame)
@@ -337,6 +338,7 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
 
     """
     from pandas.core.indexes.datetimes import DatetimeIndex
+    from pandas import DataFrame
 
     tz = 'utc' if utc else None
 
@@ -446,9 +448,24 @@ def _convert_listlike(arg, box, format, name=None, tz=tz):
             except (ValueError, TypeError):
                 raise e
 
+    def check_numerical_arg():
+        return ((is_scalar(arg) and (is_integer(arg) or is_float(arg))) or
+                is_numeric_dtype(np.asarray(arg)))
+
     if arg is None:
         return None
 
+    if unit is not None:
+        if isinstance(arg, (DataFrame,)):
+            raise ValueError("unit must be None if arg is a DataFrame")
+        if is_bool_dtype(np.asarray(arg)):
+            raise TypeError("{0} is not convertible to datetime"
+                            .format(type(arg)))
+    else:
+        if (format is None and check_numerical_arg() and
+        not isinstance(arg, (DataFrame,))):
+            raise ValueError("a unit is required in case of numerical arg")
+
     # handle origin
     if origin == 'julian':
 
@@ -474,8 +491,7 @@ def _convert_listlike(arg, box, format, name=None, tz=tz):
 
         # arg must be a numeric
         original = arg
-        if not ((is_scalar(arg) and (is_integer(arg) or is_float(arg))) or
-                is_numeric_dtype(np.asarray(arg))):
+        if not check_numerical_arg():
             raise ValueError(
                 "'{arg}' is not compatible with origin='{origin}'; "
                 "it must be numeric with a unit specified ".format(