Fix recursive brace expansion and enable test case

The solution is a copy from
https://rosettacode.org/wiki/Brace_expansion#Python
with some minor adaptions to clearer variable names and at least a sentence
describing the functionality.

root_pandas/readwrite.py

@@ -27,27 +27,57 @@
 NOEXPAND_PREFIX = 'noexpand:'
 
 
-def expand_braces(orig):
-    r = r'.*?(\{.+[^\\]\})'
-    p = re.compile(r)
-
-    s = orig[:]
-    res = list()
-
-    m = p.search(s)
-    if m is not None:
-        sub = m.group(1)
-        open_brace = s.find(sub)
-        close_brace = open_brace + len(sub) - 1
-        if sub.find(',') != -1:
-            for pat in sub[1:-1].split(','):
-                res.extend(expand_braces(s[:open_brace] + pat + s[close_brace+1:]))
-        else:
-            res.extend(expand_braces(s[:open_brace] + sub.replace('}', '\\}') + s[close_brace+1:]))
-    else:
-        res.append(s.replace('\\}', '}'))
+def _getitem(string, depth=0):
+    """
+    Get an item from the string (where item is up to the next ',' or '}' or the
+    end of the string)
+    """
+    out = [""]
+    while string:
+        char = string[0]
+        if depth and (char == ',' or char == '}'):
+            return out, string
+        if char == '{':
+            groups_string = _getgroup(string[1:], depth+1)
+            if groups_string is not None:
+                groups, string = groups_string
+                out = [a + g for a in out for g in groups]
+                continue
+        if char == '\\' and len(string) > 1:
+            string, char = string[1:], char + string[1]
+
+        out, string = [a + char for a in out], string[1:]
+
+    return out, string
+
+
+def _getgroup(string, depth):
+    """
+    Get a group from the string, where group is a list of all the comma
+    separated substrings up to the next '}' char or the brace enclosed substring
+    if there is no comma
+    """
+    out, comma = [], False
+    while string:
+        items, string = _getitem(string, depth)
 
-    return list(set(res))
+        if not string:
+            break
+        out += items
+
+        if string[0] == '}':
+            if comma:
+                return out, string[1:]
+            return ['{' + a + '}' for a in out], string[1:]
+
+        if string[0] == ',':
+            comma, string = True, string[1:]
+
+    return None
+
+
+def expand_braces(orig):
+    return _getitem(orig, 0)[0]
 
 
 def get_nonscalar_columns(array):

tests/test.py

@@ -304,10 +304,10 @@ def test_brace_pattern_in_columns():
     assert_frame_equal(df[['var{03}', 'var2', 'var{04}']],
                        reference_df[['var{03}', 'var2', 'var{04}']])
 
-    # # TODO Recursive expansions
-    # df = read_root('tmp.root', columns=[r'var{0{2,3},1{1,3}}'])
-    # assert set(df.columns) == {'var02', 'var03', 'var11', 'var13'}
-    # assert_frame_equal(df[['var02', 'var03', 'var11', 'var13']],
-    #                    reference_df[['var02', 'var03', 'var11', 'var13']])
+    # Recursive expansions
+    df = read_root('tmp.root', columns=[r'var{0{2,3},1{1,3}}'])
+    assert set(df.columns) == {'var02', 'var03', 'var11', 'var13'}
+    assert_frame_equal(df[['var02', 'var03', 'var11', 'var13']],
+                       reference_df[['var02', 'var03', 'var11', 'var13']])
 
     os.remove('tmp.root')