Add a pure_alias dict containing Y->A

Author: Iva Miholic

pandas/tseries/frequencies.py

@@ -421,6 +421,26 @@ def get_period_alias(offset_str):
     """ alias to closest period strings BQ->Q etc"""
     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',
@@ -718,6 +738,7 @@ 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: