BUG: Don't allow extra fields to be added in genfromtxt

Previously passing `dtype=[], names=['a']` would append an extra field, even though `dtype=['a'], names=['b', 'c']` does not.
numpy · charris · Aug 23, 2019 · Aug 16, 2019 · Aug 17, 2019 · Aug 17, 2019
commit b11468e1a38c19a24ed0ebaede023c159c745e33
diff --git a/numpy/lib/_iotools.py b/numpy/lib/_iotools.py
@@ -925,28 +925,27 @@ def easy_dtype(ndtype, names=None, defaultfmt="f%i", **validationargs):
         names = validate(names, nbfields=nbfields, defaultfmt=defaultfmt)
         ndtype = np.dtype(dict(formats=ndtype, names=names))
     else:
-        nbtypes = len(ndtype)
         # Explicit names
         if names is not None:
             validate = NameValidator(**validationargs)
             if isinstance(names, basestring):
                 names = names.split(",")
             # Simple dtype: repeat to match the nb of names
-            if nbtypes == 0:
+            if ndtype.names is None:
                 formats = tuple([ndtype.type] * len(names))
                 names = validate(names, defaultfmt=defaultfmt)
                 ndtype = np.dtype(list(zip(names, formats)))
             # Structured dtype: just validate the names as needed
             else:
-                ndtype.names = validate(names, nbfields=nbtypes,
+                ndtype.names = validate(names, nbfields=len(ndtype.names),
                                         defaultfmt=defaultfmt)
         # No implicit names
-        elif (nbtypes > 0):
+        elif ndtype.names is not None:
             validate = NameValidator(**validationargs)
             # Default initial names : should we change the format ?
-            if ((ndtype.names == tuple("f%i" % i for i in range(nbtypes))) and
+            if ((ndtype.names == tuple("f%i" % i for i in range(len(ndtype.names)))) and
                     (defaultfmt != "f%i")):
-                ndtype.names = validate([''] * nbtypes, defaultfmt=defaultfmt)
+                ndtype.names = validate([''] * len(ndtype.names), defaultfmt=defaultfmt)
             # Explicit initial names : just validate
             else:
                 ndtype.names = validate(ndtype.names, defaultfmt=defaultfmt)

diff --git a/numpy/lib/npyio.py b/numpy/lib/npyio.py
@@ -2147,7 +2147,7 @@ def encode_unicode_cols(row_tup):
             outputmask = np.array(masks, dtype=mdtype)
     else:
         # Overwrite the initial dtype names if needed
-        if names and dtype.names:
+        if names and dtype.names is not None:
             dtype.names = names
         # Case 1. We have a structured type
         if len(dtype_flat) > 1: