Make pd.Period immutable (pandas-dev#17239)

Author: jbrockmendel

doc/source/whatsnew/v0.21.0.txt

@@ -292,6 +292,8 @@ Other API Changes
 - Moved definition of ``MergeError`` to the ``pandas.errors`` module.
 - The signature of :func:`Series.set_axis` and :func:`DataFrame.set_axis` has been changed from ``set_axis(axis, labels)`` to ``set_axis(labels, axis=0)``, for consistency with the rest of the API. The old signature is deprecated and will show a ``FutureWarning`` (:issue:`14636`)
 - :func:`Series.argmin` and :func:`Series.argmax` will now raise a ``TypeError`` when used with ``object`` dtypes, instead of a ``ValueError`` (:issue:`13595`)
+- :class:`Period` is now immutable, and will now raise an ``AttributeError`` when a user tries to assign a new value to the ``ordinal`` or ``freq`` attributes (:issue:`17116`).
+
 
 .. _whatsnew_0210.deprecations:
 

pandas/_libs/period.pyx

@@ -29,7 +29,9 @@ from datetime cimport (
     PANDAS_FR_ns,
     INT32_MIN)
 
+
 cimport util, lib
+
 from lib cimport is_null_datetimelike, is_period
 from pandas._libs import tslib, lib
 from pandas._libs.tslib import (Timedelta, Timestamp, iNaT,
@@ -668,13 +670,17 @@ class IncompatibleFrequency(ValueError):
 
 cdef class _Period(object):
 
-    cdef public:
+    cdef readonly:
         int64_t ordinal
         object freq
 
     _comparables = ['name', 'freqstr']
     _typ = 'period'
 
+    def __cinit__(self, ordinal, freq):
+        self.ordinal = ordinal
+        self.freq = freq
+
     @classmethod
     def _maybe_convert_freq(cls, object freq):
 
@@ -698,9 +704,8 @@ cdef class _Period(object):
         if ordinal == iNaT:
             return NaT
         else:
-            self = _Period.__new__(cls)
-            self.ordinal = ordinal
-            self.freq = cls._maybe_convert_freq(freq)
+            freq = cls._maybe_convert_freq(freq)
+            self = _Period.__new__(cls, ordinal, freq)
             return self
 
     def __richcmp__(self, other, op):
@@ -752,7 +757,7 @@ cdef class _Period(object):
     def __add__(self, other):
         if isinstance(self, Period):
             if isinstance(other, (timedelta, np.timedelta64,
-                                  offsets.Tick, offsets.DateOffset,
+                                  offsets.DateOffset,
                                   Timedelta)):
                 return self._add_delta(other)
             elif other is NaT:
@@ -770,7 +775,7 @@ cdef class _Period(object):
     def __sub__(self, other):
         if isinstance(self, Period):
             if isinstance(other, (timedelta, np.timedelta64,
-                                  offsets.Tick, offsets.DateOffset,
+                                  offsets.DateOffset,
                                   Timedelta)):
                 neg_other = -other
                 return self + neg_other

pandas/tests/scalar/test_period.py

@@ -1406,3 +1406,14 @@ def test_period_ops_offset(self):
 
         with tm.assert_raises_regex(period.IncompatibleFrequency, msg):
             p - offsets.Hour(2)
+
+
+def test_period_immutable():
+    # see gh-17116
+    per = pd.Period('2014Q1')
+    with pytest.raises(AttributeError):
+        per.ordinal = 14
+
+    freq = per.freq
+    with pytest.raises(AttributeError):
+        per.freq = 2 * freq