Updated documentation indicating default behaviour is to strip whitespace, and how to override. Enhances GH-issue-16950 pandas-dev#16950

Author: RonaldBarnes

doc/source/user_guide/io.rst

@@ -1366,8 +1366,10 @@ a different usage of the ``delimiter`` parameter:
 * ``widths``: A list of field widths which can be used instead of 'colspecs'
   if the intervals are contiguous.
 * ``delimiter``: Characters to consider as filler characters in the fixed-width file.
-  Can be used to specify the filler character of the fields
-  if it is not spaces (e.g., '~').
+  Default is "`` \t``"  (space and tab).
+  Used to specify the character(s) to strip from start and end of every field.
+  To preserve whitespace, set to a character that does not exist in the data,
+  i.e. "\0".
 
 Consider a typical fixed-width data file:
 
@@ -1404,8 +1406,9 @@ column widths for contiguous columns:
    df = pd.read_fwf("bar.csv", widths=widths, header=None)
    df
 
-The parser will take care of extra white spaces around the columns
+The parser will take care of extra whitespace around the columns,
 so it's ok to have extra separation between the columns in the file.
+To preserve whitespace around the columns, see ``delimiter``.
 
 By default, ``read_fwf`` will try to infer the file's ``colspecs`` by using the
 first 100 rows of the file. It can do it only in cases when the columns are

pandas/io/parsers/readers.py

@@ -1231,6 +1231,7 @@ def read_fwf(
     *,
     colspecs: Sequence[tuple[int, int]] | str | None = "infer",
     widths: Sequence[int] | None = None,
+    delimiter: str | None = " \t",
     infer_nrows: int = 100,
     **kwds,
 ) -> DataFrame | TextFileReader:
@@ -1251,7 +1252,7 @@ def read_fwf(
         Valid URL schemes include http, ftp, s3, and file. For file URLs, a host is
         expected. A local file could be:
         ``file://localhost/path/to/table.csv``.
-    colspecs : list of tuple (int, int) or 'infer'. optional
+    colspecs : list of tuple (int, int) or 'infer', optional
         A list of tuples giving the extents of the fixed-width
         fields of each line as half-open intervals (i.e.,  [from, to[ ).
         String value 'infer' can be used to instruct the parser to try
@@ -1260,6 +1261,9 @@ def read_fwf(
     widths : list of int, optional
         A list of field widths which can be used instead of 'colspecs' if
         the intervals are contiguous.
+    delimiter : str, default " \t" (space and tab), optional
+        Character(s) to strip from start and end of each field. To
+        preserve whitespace, must be non-default value (i.e. delimiter="\0").
     infer_nrows : int, default 100
         The number of rows to consider when letting the parser determine the
         `colspecs`.