ENH: Improve performance of np.save for small arrays (gh-18657)

* ENH: Remove call to `_filter_header` from `_write_array_header`

Improve performance of `np.save` by removing the call
when writing the header, as it is known to be done in Python 3.

* ENH: Only call `_filter_header` from `_read_array_header` for old vers

Improve performance of `np.load` for arrays with version >= (3,0)
by removing the call, as it is known to be done in Python 3.

* ENH: Use a set of keys when checking `read_array`

Improve performance of `np.load`.

* DOC: Improve performance of `np.{save,load}` for small arrays

doc/release/upcoming_changes/18657.performance.rst

@@ -0,0 +1,10 @@
+Improve performance of ``np.save`` and ``np.load`` for small arrays
+-------------------------------------------------------------------
+``np.save`` is now a lot faster for small arrays.
+
+``np.load`` is also faster for small arrays,
+but only when serializing with a version >= `(3, 0)`.
+
+Both are done by removing checks that are only relevant for Python 2,
+while still maintaining compatibility with arrays
+which might have been created by Python 2.

numpy/lib/format.py

@@ -173,6 +173,7 @@
 __all__ = []
 
 
+EXPECTED_KEYS = {'descr', 'fortran_order', 'shape'}
 MAGIC_PREFIX = b'\x93NUMPY'
 MAGIC_LEN = len(MAGIC_PREFIX) + 2
 ARRAY_ALIGN = 64 # plausible values are powers of 2 between 16 and 4096
@@ -432,7 +433,6 @@ def _write_array_header(fp, d, version=None):
         header.append("'%s': %s, " % (key, repr(value)))
     header.append("}")
     header = "".join(header)
-    header = _filter_header(header)
     if version is None:
         header = _wrap_header_guess_version(header)
     else:
@@ -590,7 +590,10 @@ def _read_array_header(fp, version):
     #   "shape" : tuple of int
     #   "fortran_order" : bool
     #   "descr" : dtype.descr
-    header = _filter_header(header)
+    # Versions (2, 0) and (1, 0) could have been created by a Python 2
+    # implementation before header filtering was implemented.
+    if version <= (2, 0):
+        header = _filter_header(header)
     try:
         d = safe_eval(header)
     except SyntaxError as e:
@@ -599,14 +602,15 @@ def _read_array_header(fp, version):
     if not isinstance(d, dict):
         msg = "Header is not a dictionary: {!r}"
         raise ValueError(msg.format(d))
-    keys = sorted(d.keys())
-    if keys != ['descr', 'fortran_order', 'shape']:
+
+    if EXPECTED_KEYS != d.keys():
+        keys = sorted(d.keys())
         msg = "Header does not contain the correct keys: {!r}"
-        raise ValueError(msg.format(keys))
+        raise ValueError(msg.format(d.keys()))
 
     # Sanity-check the values.
     if (not isinstance(d['shape'], tuple) or
-            not numpy.all([isinstance(x, int) for x in d['shape']])):
+            not all(isinstance(x, int) for x in d['shape'])):
         msg = "shape is not valid: {!r}"
         raise ValueError(msg.format(d['shape']))
     if not isinstance(d['fortran_order'], bool):