Fix headers for list of dicts

README.rst

@@ -87,6 +87,31 @@ If ``headers="firstrow"``, then the first row of data is used::
     Alice      24
     Bob        19
 
+When the data is a list of dict, ``headers="firstrow"`` can also be 
+used to assign different column names to each corresponding dict key:
+
+        >>> print tabulate([{"name": "Name", "age": "Age"}, 
+                            {"name": "Alice", "age": 24},
+                            {"name": "Bob", "age": 19}],
+        ...                headers="firstrow")
+    Name      Age
+    ------  -----
+    Alice      24
+    Bob        19
+
+Furthermore with a list of dict, you can also specify ``headers`` as 
+a dict (similar to the example above), or as a list of keys. Either way,
+the specified keys can be a subset of all the keys present across the 
+dataset:
+
+        >>> print tabulate([{"foo": 1, "bar": 2},
+                            {"foo": 3, "bar": 4, "baz": 5}],
+        ...                headers=["bar","foo"])
+    bar    foo
+    -----  -----
+        2      1
+        4      3
+
 
 If ``headers="keys"``, then the keys of a dictionary/dataframe, or
 column indices are used. It also works for NumPy record arrays and

tabulate.py

@@ -17,6 +17,7 @@
     _float_type = float
     _text_type = unicode
     _binary_type = str
+    _string_type = basestring
 else:
     from itertools import zip_longest as izip_longest
     from functools import reduce, partial
@@ -25,6 +26,7 @@
     _float_type = float
     _text_type = str
     _binary_type = bytes
+    _string_type = str
 
 
 __all__ = ["tabulate", "tabulate_formats", "simple_separated_format"]
@@ -527,24 +529,31 @@ def _normalize_tabular_data(tabular_data, headers):
         elif (len(rows) > 0
               and isinstance(rows[0], dict)):
             # dict or OrderedDict
-            uniq_keys = set() # implements hashed lookup
-            keys = [] # storage for set
+
             if headers == "firstrow":
-                firstdict = rows[0] if len(rows) > 0 else {}
-                keys.extend(firstdict.keys())
-                uniq_keys.update(keys)
+                headers = rows[0] if len(rows) > 0 else {}
                 rows = rows[1:]
-            for row in rows:
-                for k in row.keys():
-                    #Save unique items in input order
-                    if k not in uniq_keys:
-                        keys.append(k)
-                        uniq_keys.add(k)
+
+            if isinstance(headers,_string_type):
+                # list unique keys in input order
+                uniq_keys = set() # implements hashed lookup
+                keys = [] # storage for set
+                for row in rows:
+                    for k in row.keys():
+                        if k not in uniq_keys:
+                            keys.append(k)
+                            uniq_keys.add(k)
+            elif hasattr(headers,'keys') and hasattr(headers,'values'):
+                # dict-like { key => header name }
+                keys = list(headers.keys())
+                headers = list(headers.values())
+            else:
+                # list-like [key1, key2, ...]
+                keys = list(headers) 
+
             if headers == 'keys':
                 headers = keys
-            elif headers == "firstrow" and len(rows) > 0:
-                headers = [firstdict.get(k, k) for k in keys]
-                headers = list(map(_text_type, headers))
+
             rows = [[row.get(k) for k in keys] for row in rows]
         elif headers == "keys" and len(rows) > 0:
             # keys are column indices
@@ -560,10 +569,10 @@ def _normalize_tabular_data(tabular_data, headers):
 
     # pad with empty headers for initial columns if necessary
     if headers and len(rows) > 0:
-       nhs = len(headers)
+       nhead = len(headers)
        ncols = len(rows[0])
-       if nhs < ncols:
-           headers = [""]*(ncols - nhs) + headers
+       if nhead != ncols:
+           raise RuntimeError('Number of headers and columns do not match.')
 
     return rows, headers
 
@@ -878,4 +887,4 @@ def _format_table(fmt, headers, rows, colwidths, colaligns):
     if fmt.linebelow and "linebelow" not in hidden:
         lines.append(_build_line(padded_widths, colaligns, fmt.linebelow))
 
-    return "\n".join(lines)
+    return "\n".join(lines)