Repository: pandas-dev/pandas

Issue:
ENH date_range yearly frequency flag 'A' is confusing, 'Y' should be accepted
to get a yearly frequency from date range the flag is actually `freq="A"` (for "Annual" i guess) rather than `freq="Y"` for yearly.

It would seem more consistent if it accepted "Y" as well/instead of "A", following the pattern:

hour -> hourly -> H
day -> daily -> D
week -> weekly -> W
year -> yearly -> Y



Discussion:
Anecdotally, I do `freq='Y'` almost every time. Same for resample.

The referenced issue, https://github.com/pandas-dev/pandas/issues/9312#issuecomment-71404429, concludes `freq=Y` should be an alias for `freq='AS'`
Hi, 
The aliases are here:
 https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/frequencies.py#L425

are we fine with `freq='Y'` being an alias for the beginning of the year while `freq='A'` being an alias for the end of the year? 

Additionally, should other composed freqs containing `'Y'` ( 'BYA', 'BYE', 'YA', 'YE') also be added? 
so these should following the same convention as Q and M

so Y would be the same as A e.g. end of year and

YS would be year start

Resolving patch:
diff --git a/pandas/tseries/frequencies.py b/pandas/tseries/frequencies.py
--- a/pandas/tseries/frequencies.py
+++ b/pandas/tseries/frequencies.py
@@ -422,6 +422,27 @@ def get_period_alias(offset_str):
     return _offset_to_period_map.get(offset_str, None)
 
 
+_pure_alias = {
+    # 'A' is equivalent to 'Y'.
+    'Y': 'A',
+    'YS': 'AS',
+    'BY': 'BA',
+    'BYS': 'BAS',
+    'Y-DEC': 'A-DEC',
+    'Y-JAN': 'A-JAN',
+    'Y-FEB': 'A-FEB',
+    'Y-MAR': 'A-MAR',
+    'Y-APR': 'A-APR',
+    'Y-MAY': 'A-MAY',
+    'Y-JUN': 'A-JUN',
+    'Y-JUL': 'A-JUL',
+    'Y-AUG': 'A-AUG',
+    'Y-SEP': 'A-SEP',
+    'Y-OCT': 'A-OCT',
+    'Y-NOV': 'A-NOV',
+}
+
+
 _lite_rule_alias = {
     'W': 'W-SUN',
     'Q': 'Q-DEC',
@@ -718,6 +739,7 @@ def get_standard_freq(freq):
 
 
 def _period_str_to_code(freqstr):
+    freqstr = _pure_alias.get(freqstr, freqstr)
     freqstr = _lite_rule_alias.get(freqstr, freqstr)
 
     if freqstr not in _dont_uppercase:


Test patch:
diff --git a/pandas/tests/tseries/test_frequencies.py b/pandas/tests/tseries/test_frequencies.py
--- a/pandas/tests/tseries/test_frequencies.py
+++ b/pandas/tests/tseries/test_frequencies.py
@@ -248,9 +248,10 @@ def test_anchored_shortcuts(self):
 
         # ensure invalid cases fail as expected
         invalid_anchors = ['SM-0', 'SM-28', 'SM-29',
-                           'SM-FOO', 'BSM', 'SM--1'
+                           'SM-FOO', 'BSM', 'SM--1',
                            'SMS-1', 'SMS-28', 'SMS-30',
-                           'SMS-BAR', 'BSMS', 'SMS--2']
+                           'SMS-BAR', 'SMS-BYR' 'BSMS',
+                           'SMS--2']
         for invalid_anchor in invalid_anchors:
             with tm.assert_raises_regex(ValueError,
                                         'Invalid frequency: '):
@@ -292,11 +293,15 @@ def test_get_rule_month():
 
     result = frequencies._get_rule_month('A-DEC')
     assert (result == 'DEC')
+    result = frequencies._get_rule_month('Y-DEC')
+    assert (result == 'DEC')
     result = frequencies._get_rule_month(offsets.YearEnd())
     assert (result == 'DEC')
 
     result = frequencies._get_rule_month('A-MAY')
     assert (result == 'MAY')
+    result = frequencies._get_rule_month('Y-MAY')
+    assert (result == 'MAY')
     result = frequencies._get_rule_month(offsets.YearEnd(month=5))
     assert (result == 'MAY')
 
@@ -305,6 +310,10 @@ def test_period_str_to_code():
     assert (frequencies._period_str_to_code('A') == 1000)
     assert (frequencies._period_str_to_code('A-DEC') == 1000)
     assert (frequencies._period_str_to_code('A-JAN') == 1001)
+    assert (frequencies._period_str_to_code('Y') == 1000)
+    assert (frequencies._period_str_to_code('Y-DEC') == 1000)
+    assert (frequencies._period_str_to_code('Y-JAN') == 1001)
+
     assert (frequencies._period_str_to_code('Q') == 2000)
     assert (frequencies._period_str_to_code('Q-DEC') == 2000)
     assert (frequencies._period_str_to_code('Q-FEB') == 2002)
@@ -349,6 +358,10 @@ def test_freq_code(self):
         assert frequencies.get_freq('3A') == 1000
         assert frequencies.get_freq('-1A') == 1000
 
+        assert frequencies.get_freq('Y') == 1000
+        assert frequencies.get_freq('3Y') == 1000
+        assert frequencies.get_freq('-1Y') == 1000
+
         assert frequencies.get_freq('W') == 4000
         assert frequencies.get_freq('W-MON') == 4001
         assert frequencies.get_freq('W-FRI') == 4005
@@ -369,6 +382,13 @@ def test_freq_group(self):
         assert frequencies.get_freq_group('-1A') == 1000
         assert frequencies.get_freq_group('A-JAN') == 1000
         assert frequencies.get_freq_group('A-MAY') == 1000
+
+        assert frequencies.get_freq_group('Y') == 1000
+        assert frequencies.get_freq_group('3Y') == 1000
+        assert frequencies.get_freq_group('-1Y') == 1000
+        assert frequencies.get_freq_group('Y-JAN') == 1000
+        assert frequencies.get_freq_group('Y-MAY') == 1000
+
         assert frequencies.get_freq_group(offsets.YearEnd()) == 1000
         assert frequencies.get_freq_group(offsets.YearEnd(month=1)) == 1000
         assert frequencies.get_freq_group(offsets.YearEnd(month=5)) == 1000
@@ -790,12 +810,6 @@ def test_series(self):
         for freq in [None, 'L']:
             s = Series(period_range('2013', periods=10, freq=freq))
             pytest.raises(TypeError, lambda: frequencies.infer_freq(s))
-        for freq in ['Y']:
-
-            msg = frequencies._INVALID_FREQ_ERROR
-            with tm.assert_raises_regex(ValueError, msg):
-                s = Series(period_range('2013', periods=10, freq=freq))
-            pytest.raises(TypeError, lambda: frequencies.infer_freq(s))
 
         # DateTimeIndex
         for freq in ['M', 'L', 'S']:
@@ -812,11 +826,12 @@ def test_legacy_offset_warnings(self):
                  'W@FRI', 'W@SAT', 'W@SUN', 'Q@JAN', 'Q@FEB', 'Q@MAR',
                  'A@JAN', 'A@FEB', 'A@MAR', 'A@APR', 'A@MAY', 'A@JUN',
                  'A@JUL', 'A@AUG', 'A@SEP', 'A@OCT', 'A@NOV', 'A@DEC',
-                 'WOM@1MON', 'WOM@2MON', 'WOM@3MON', 'WOM@4MON',
-                 'WOM@1TUE', 'WOM@2TUE', 'WOM@3TUE', 'WOM@4TUE',
-                 'WOM@1WED', 'WOM@2WED', 'WOM@3WED', 'WOM@4WED',
-                 'WOM@1THU', 'WOM@2THU', 'WOM@3THU', 'WOM@4THU'
-                 'WOM@1FRI', 'WOM@2FRI', 'WOM@3FRI', 'WOM@4FRI']
+                 'Y@JAN', 'WOM@1MON', 'WOM@2MON', 'WOM@3MON',
+                 'WOM@4MON', 'WOM@1TUE', 'WOM@2TUE', 'WOM@3TUE',
+                 'WOM@4TUE', 'WOM@1WED', 'WOM@2WED', 'WOM@3WED',
+                 'WOM@4WED', 'WOM@1THU', 'WOM@2THU', 'WOM@3THU',
+                 'WOM@4THU', 'WOM@1FRI', 'WOM@2FRI', 'WOM@3FRI',
+                 'WOM@4FRI']
 
         msg = frequencies._INVALID_FREQ_ERROR
         for freq in freqs: