DOC: header='infer' is not working when there is no header, closes #17473 (#18042)

(cherry picked from commit 67c4d0f)

Author: cmazzullo

doc/source/io.rst

@@ -103,15 +103,20 @@ Column and Index Locations and Names
 ++++++++++++++++++++++++++++++++++++
 
 header : int or list of ints, default ``'infer'``
-  Row number(s) to use as the column names, and the start of the data. Default
-  behavior is as if ``header=0`` if no ``names`` passed, otherwise as if
-  ``header=None``. Explicitly pass ``header=0`` to be able to replace existing
-  names. The header can be a list of ints that specify row locations for a
-  multi-index on the columns e.g. ``[0,1,3]``. Intervening rows that are not
-  specified will be skipped (e.g. 2 in this example is skipped). Note that
-  this parameter ignores commented lines and empty lines if
-  ``skip_blank_lines=True``, so header=0 denotes the first line of data
-  rather than the first line of the file.
+  Row number(s) to use as the column names, and the start of the
+  data. Default behavior is to infer the column names: if no names are
+  passed the behavior is identical to ``header=0`` and column names
+  are inferred from the first line of the file, if column names are
+  passed explicitly then the behavior is identical to
+  ``header=None``. Explicitly pass ``header=0`` to be able to replace
+  existing names.
+
+  The header can be a list of ints that specify row locations
+  for a multi-index on the columns e.g. ``[0,1,3]``. Intervening rows
+  that are not specified will be skipped (e.g. 2 in this example is
+  skipped). Note that this parameter ignores commented lines and empty
+  lines if ``skip_blank_lines=True``, so header=0 denotes the first
+  line of data rather than the first line of the file.
 names : array-like, default ``None``
   List of column names to use. If file contains no header row, then you should
   explicitly pass ``header=None``. Duplicates in this list will cause
@@ -553,6 +558,14 @@ If the header is in a row other than the first, pass the row number to
     data = 'skip this skip it\na,b,c\n1,2,3\n4,5,6\n7,8,9'
     pd.read_csv(StringIO(data), header=1)
 
+.. note::
+
+  Default behavior is to infer the column names: if no names are
+  passed the behavior is identical to ``header=0`` and column names
+  are inferred from the first nonblank line of the file, if column
+  names are passed explicitly then the behavior is identical to
+  ``header=None``.
+
 .. _io.dupe_names:
 
 Duplicate names parsing

pandas/io/parsers.py

@@ -74,15 +74,19 @@
     .. versionadded:: 0.18.1 support for the Python parser.
 
 header : int or list of ints, default 'infer'
-    Row number(s) to use as the column names, and the start of the data.
-    Default behavior is as if set to 0 if no ``names`` passed, otherwise
-    ``None``. Explicitly pass ``header=0`` to be able to replace existing
-    names. The header can be a list of integers that specify row locations for
-    a multi-index on the columns e.g. [0,1,3]. Intervening rows that are not
-    specified will be skipped (e.g. 2 in this example is skipped). Note that
-    this parameter ignores commented lines and empty lines if
-    ``skip_blank_lines=True``, so header=0 denotes the first line of data
-    rather than the first line of the file.
+    Row number(s) to use as the column names, and the start of the
+    data.  Default behavior is to infer the column names: if no names
+    are passed the behavior is identical to ``header=0`` and column
+    names are inferred from the first line of the file, if column
+    names are passed explicitly then the behavior is identical to
+    ``header=None``. Explicitly pass ``header=0`` to be able to
+    replace existing names. The header can be a list of integers that
+    specify row locations for a multi-index on the columns
+    e.g. [0,1,3]. Intervening rows that are not specified will be
+    skipped (e.g. 2 in this example is skipped). Note that this
+    parameter ignores commented lines and empty lines if
+    ``skip_blank_lines=True``, so header=0 denotes the first line of
+    data rather than the first line of the file.
 names : array-like, default None
     List of column names to use. If file contains no header row, then you
     should explicitly pass header=None. Duplicates in this list will cause