change default encoding for stringtochar, chartostring to ascii for dtype='S' (issue #1464)

Author: jswhit2

examples/tutorial.py

@@ -163,6 +163,7 @@ def walktree(top):
 datac2.imag = datain['imag']
 print(datac.dtype,datac)
 print(datac2.dtype,datac2)
+nc.close()
 
 # more complex compound type example.
 nc = Dataset('compound_example.nc','w') # create a new dataset.

src/netCDF4/_netCDF4.pyx

@@ -6788,7 +6788,7 @@ returns a rank 1 numpy character array of length NUMCHARS with datatype `'S1'`
     arr[0:len(string)] = tuple(string)
     return arr
 
-def stringtochar(a,encoding='utf-8',n_strlen=None):
+def stringtochar(a,encoding=None,n_strlen=None):
     """
 **`stringtochar(a,encoding='utf-8',n_strlen=None)`**
 
@@ -6809,10 +6809,15 @@ used to represent each string in the input array).
 returns a numpy character array with datatype `'S1'` or `'U1'`
 and shape `a.shape + (N,)`, where N is the length of each string in a."""
     dtype = a.dtype.kind
-    if n_strlen is None:
-        n_strlen = a.dtype.itemsize
     if dtype not in ["S","U"]:
         raise ValueError("type must string or unicode ('S' or 'U')")
+    if encoding is None:
+       if dtype == 'S':
+           encoding = 'ascii'
+       else:
+           encoding = 'utf-8'
+    if n_strlen is None:
+        n_strlen = a.dtype.itemsize
     if encoding in ['none','None','bytes']:
         b = numpy.array(tuple(a.tobytes()),'S1')
     elif encoding == 'ascii':
@@ -6827,7 +6832,7 @@ and shape `a.shape + (N,)`, where N is the length of each string in a."""
         b = numpy.array([[bb[i:i+1] for i in range(n_strlen)] for bb in bbytes])
     return b
 
-def chartostring(b,encoding='utf-8'):
+def chartostring(b,encoding=None):
     """
 **`chartostring(b,encoding='utf-8')`**
 
@@ -6846,6 +6851,11 @@ returns a numpy string array with datatype `'UN'` (or `'SN'`) and shape
     dtype = b.dtype.kind
     if dtype not in ["S","U"]:
         raise ValueError("type must be string or unicode ('S' or 'U')")
+    if encoding is None:
+       if dtype == 'S':
+           encoding = 'ascii'
+       else:
+           encoding = 'utf-8'
     bs = b.tobytes()
     slen = int(b.shape[-1])
     if encoding in ['none','None','bytes']: