ENH: Add spaces after punctuation in dtype repr/str. (#19686)

Before:
```
In [1]: np.dtype({"names": ["a"], "formats": [int], "offsets": [2]})
Out[1]: dtype({'names':['a'], 'formats':['<i8'], 'offsets':[2], 'itemsize':10})
```

After:
```
In [1]: np.dtype({"names": ["a"], "formats": [int], "offsets": [2]})
Out[1]: dtype({'names': ['a'], 'formats': ['<i8'], 'offsets': [2], 'itemsize': 10})
```

* Allow switching back to old dtype printing format.

* Add changelog.

1 files changed, 67 insertions, 31 deletions

diff --git a/numpy/core/arrayprint.py b/numpy/core/arrayprint.py
index 2a4bef669..d7e9bf795 100644
--- a/numpy/core/arrayprint.py
+++ b/numpy/core/arrayprint.py
@@ -24,6 +24,7 @@ __docformat__ = 'restructuredtext'
 
 import functools
 import numbers
+import sys
 try:
     from _thread import get_ident
 except ImportError:
@@ -56,12 +57,17 @@ _format_options = {
     'infstr': 'inf',
     'sign': '-',
     'formatter': None,
-    'legacy': False}
+    # Internally stored as an int to simplify comparisons; converted from/to
+    # str/False on the way in/out.
+    'legacy': sys.maxsize}
 
 def _make_options_dict(precision=None, threshold=None, edgeitems=None,
                        linewidth=None, suppress=None, nanstr=None, infstr=None,
                        sign=None, formatter=None, floatmode=None, legacy=None):
-    """ make a dictionary out of the non-None arguments, plus sanity checks """
+    """
+    Make a dictionary out of the non-None arguments, plus conversion of
+    *legacy* and sanity checks.
+    """
 
     options = {k: v for k, v in locals().items() if v is not None}
 
@@ -76,9 +82,18 @@ def _make_options_dict(precision=None, threshold=None, edgeitems=None,
     if sign not in [None, '-', '+', ' ']:
         raise ValueError("sign option must be one of ' ', '+', or '-'")
 
-    if legacy not in [None, False, '1.13']:
-        warnings.warn("legacy printing option can currently only be '1.13' or "
-                      "`False`", stacklevel=3)
+    if legacy == False:
+        options['legacy'] = sys.maxsize
+    elif legacy == '1.13':
+        options['legacy'] = 113
+    elif legacy == '1.21':
+        options['legacy'] = 121
+    elif legacy is None:
+        pass  # OK, do nothing.
+    else:
+        warnings.warn(
+            "legacy printing option can currently only be '1.13', '1.21', or "
+            "`False`", stacklevel=3)
 
     if threshold is not None:
         # forbid the bad threshold arg suggested by stack overflow, gh-12351
@@ -186,11 +201,21 @@ def set_printoptions(precision=None, threshold=None, edgeitems=None,
     legacy : string or `False`, optional
         If set to the string `'1.13'` enables 1.13 legacy printing mode. This
         approximates numpy 1.13 print output by including a space in the sign
-        position of floats and different behavior for 0d arrays. If set to
-        `False`, disables legacy mode. Unrecognized strings will be ignored
-        with a warning for forward compatibility.
+        position of floats and different behavior for 0d arrays. This also
+        enables 1.21 legacy printing mode (described below).
+
+        If set to the string `'1.21'` enables 1.21 legacy printing mode. This
+        approximates numpy 1.21 print output of complex structured dtypes
+        by not inserting spaces after commas that separate fields and after
+        colons.
+
+        If set to `False`, disables legacy mode.
+
+        Unrecognized strings will be ignored with a warning for forward
+        compatibility.
 
         .. versionadded:: 1.14.0
+        .. versionchanged:: 1.22.0
 
     See Also
     --------
@@ -257,11 +282,13 @@ def set_printoptions(precision=None, threshold=None, edgeitems=None,
     _format_options.update(opt)
 
     # set the C variable for legacy mode
-    if _format_options['legacy'] == '1.13':
+    if _format_options['legacy'] == 113:
         set_legacy_print_mode(113)
         # reset the sign option in legacy mode to avoid confusion
         _format_options['sign'] = '-'
-    elif _format_options['legacy'] is False:
+    elif _format_options['legacy'] == 121:
+        set_legacy_print_mode(121)
+    elif _format_options['legacy'] == sys.maxsize:
         set_legacy_print_mode(0)
 
 
@@ -292,7 +319,16 @@ def get_printoptions():
     set_printoptions, printoptions, set_string_function
 
     """
-    return _format_options.copy()
+    opts = _format_options.copy()
+    opts['legacy'] = {
+        113: '1.13', 121: '1.21', sys.maxsize: False,
+    }[opts['legacy']]
+    return opts
+
+
+def _get_legacy_print_mode():
+    """Return the legacy print mode as an int."""
+    return _format_options['legacy']
 
 
 @set_module('numpy')
@@ -678,7 +714,7 @@ def array2string(a, max_line_width=None, precision=None,
     options = _format_options.copy()
     options.update(overrides)
 
-    if options['legacy'] == '1.13':
+    if options['legacy'] <= 113:
         if style is np._NoValue:
             style = repr
 
@@ -690,7 +726,7 @@ def array2string(a, max_line_width=None, precision=None,
                       " except in 1.13 'legacy' mode",
                       DeprecationWarning, stacklevel=3)
 
-    if options['legacy'] != '1.13':
+    if options['legacy'] > 113:
         options['linewidth'] -= len(suffix)
 
     # treat as a null array if any of shape elements == 0
@@ -702,7 +738,7 @@ def array2string(a, max_line_width=None, precision=None,
 
 def _extendLine(s, line, word, line_width, next_line_prefix, legacy):
     needs_wrap = len(line) + len(word) > line_width
-    if legacy != '1.13':
+    if legacy > 113:
         # don't wrap lines if it won't help
         if len(line) <= len(next_line_prefix):
             needs_wrap = False
@@ -719,7 +755,7 @@ def _extendLine_pretty(s, line, word, line_width, next_line_prefix, legacy):
     Extends line with nicely formatted (possibly multi-line) string ``word``.
     """
     words = word.splitlines()
-    if len(words) == 1 or legacy == '1.13':
+    if len(words) == 1 or legacy <= 113:
         return _extendLine(s, line, word, line_width, next_line_prefix, legacy)
 
     max_word_length = max(len(word) for word in words)
@@ -765,7 +801,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
         # when recursing, add a space to align with the [ added, and reduce the
         # length of the line by 1
         next_hanging_indent = hanging_indent + ' '
-        if legacy == '1.13':
+        if legacy <= 113:
             next_width = curr_width
         else:
             next_width = curr_width - len(']')
@@ -785,7 +821,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
         # last axis (rows) - wrap elements if they would not fit on one line
         if axes_left == 1:
             # the length up until the beginning of the separator / bracket
-            if legacy == '1.13':
+            if legacy <= 113:
                 elem_width = curr_width - len(separator.rstrip())
             else:
                 elem_width = curr_width - max(len(separator.rstrip()), len(']'))
@@ -800,7 +836,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
             if show_summary:
                 s, line = _extendLine(
                     s, line, summary_insert, elem_width, hanging_indent, legacy)
-                if legacy == '1.13':
+                if legacy <= 113:
                     line += ", "
                 else:
                     line += separator
@@ -811,7 +847,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
                     s, line, word, elem_width, hanging_indent, legacy)
                 line += separator
 
-            if legacy == '1.13':
+            if legacy <= 113:
                 # width of the separator is not considered on 1.13
                 elem_width = curr_width
             word = recurser(index + (-1,), next_hanging_indent, next_width)
@@ -830,7 +866,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
                 s += hanging_indent + nested + line_sep
 
             if show_summary:
-                if legacy == '1.13':
+                if legacy <= 113:
                     # trailing space, fixed nbr of newlines, and fixed separator
                     s += hanging_indent + summary_insert + ", \n"
                 else:
@@ -875,7 +911,7 @@ class FloatingFormat:
             sign = '+' if sign else '-'
 
         self._legacy = legacy
-        if self._legacy == '1.13':
+        if self._legacy <= 113:
             # when not 0d, legacy does not support '-'
             if data.shape != () and sign == '-':
                 sign = ' '
@@ -919,7 +955,7 @@ class FloatingFormat:
             self.min_digits = None
         elif self.exp_format:
             trim, unique = '.', True
-            if self.floatmode == 'fixed' or self._legacy == '1.13':
+            if self.floatmode == 'fixed' or self._legacy <= 113:
                 trim, unique = 'k', False
             strs = (dragon4_scientific(x, precision=self.precision,
                                unique=unique, trim=trim, sign=self.sign == '+')
@@ -934,7 +970,7 @@ class FloatingFormat:
             self.unique = unique
 
             # for back-compat with np 1.13, use 2 spaces & sign and full prec
-            if self._legacy == '1.13':
+            if self._legacy <= 113:
                 self.pad_left = 3
             else:
                 # this should be only 1 or 2. Can be calculated from sign.
@@ -951,7 +987,7 @@ class FloatingFormat:
                                        sign=self.sign == '+')
                     for x in finite_vals)
             int_part, frac_part = zip(*(s.split('.') for s in strs))
-            if self._legacy == '1.13':
+            if self._legacy <= 113:
                 self.pad_left = 1 + max(len(s.lstrip('-+')) for s in int_part)
             else:
                 self.pad_left = max(len(s) for s in int_part)
@@ -966,7 +1002,7 @@ class FloatingFormat:
                 self.trim = '.'
                 self.min_digits = 0
 
-        if self._legacy != '1.13':
+        if self._legacy > 113:
             # account for sign = ' ' by adding one to pad_left
             if self.sign == ' ' and not any(np.signbit(finite_vals)):
                 self.pad_left += 1
@@ -1215,7 +1251,7 @@ class ComplexFloatingFormat:
             sign = '+' if sign else '-'
 
         floatmode_real = floatmode_imag = floatmode
-        if legacy == '1.13':
+        if legacy <= 113:
             floatmode_real = 'maxprec_equal'
             floatmode_imag = 'maxprec'
 
@@ -1286,7 +1322,7 @@ class DatetimeFormat(_TimelikeFormat):
         super().__init__(x)
 
     def __call__(self, x):
-        if self.legacy == '1.13':
+        if self.legacy <= 113:
             return self._format_non_nat(x)
         return super().__call__(x)
 
@@ -1390,7 +1426,7 @@ def dtype_is_implied(dtype):
     array([1, 2, 3], dtype=int8)
     """
     dtype = np.dtype(dtype)
-    if _format_options['legacy'] == '1.13' and dtype.type == bool_:
+    if _format_options['legacy'] <= 113 and dtype.type == bool_:
         return False
 
     # not just void types can be structured, and names are not part of the repr
@@ -1445,7 +1481,7 @@ def _array_repr_implementation(
     prefix = class_name + "("
     suffix = ")" if skipdtype else ","
 
-    if (_format_options['legacy'] == '1.13' and
+    if (_format_options['legacy'] <= 113 and
             arr.shape == () and not arr.dtype.names):
         lst = repr(arr.item())
     elif arr.size > 0 or arr.shape == (0,):
@@ -1466,7 +1502,7 @@ def _array_repr_implementation(
     # Note: This line gives the correct result even when rfind returns -1.
     last_line_len = len(arr_str) - (arr_str.rfind('\n') + 1)
     spacer = " "
-    if _format_options['legacy'] == '1.13':
+    if _format_options['legacy'] <= 113:
         if issubclass(arr.dtype.type, flexible):
             spacer = '\n' + ' '*len(class_name + "(")
     elif last_line_len + len(dtype_str) + 1 > max_line_width:
@@ -1540,7 +1576,7 @@ def _array_str_implementation(
         a, max_line_width=None, precision=None, suppress_small=None,
         array2string=array2string):
     """Internal version of array_str() that allows overriding array2string."""
-    if (_format_options['legacy'] == '1.13' and
+    if (_format_options['legacy'] <= 113 and
             a.shape == () and not a.dtype.names):
         return str(a.item())