implement height and width fallback for non-tty

once the height and width test is resolved to actually test, we
notice that running `nosetests 2>&1 | less' & etc. will fail:
the ioctl for TIOCGWINSZ fails for non-ttys (such as used by travis CI).
So, we fall-through to the LINES, COLUMNS environment variables, with
default values of (24, 80). (24, 80) has been a fairly standard
screensize for IBM PC-DOS and Apple ][ (beginning with the 80-column
character card), and C128. It is also the default for xterm, and many
classic terminals (such as a vt220) or emulating terminals (such as
telix), or bulletin board servers (such as teleguard) where a 'status
line' is also present.  In reality, these screens are capable of *25*
lines, but the 25th line is reserved for the status line.

Irregardless, non-zero must be returned for 'height' and 'width'
properties, as a value of 0 may become a "divide by zero" error
for mathematical operations that make use of the terminal height or
width in scripts that the user may chose to pipe to 'less -r' or some
such. In these situations, even though a value is returned, operations
such as 'move(x, y)' would still become 'stripped' due to 'is_a_tty'
becoming False, so there is no actual harm in providing a terminal size
that is not legal.

1 files changed, 10 insertions, 2 deletions

diff --git a/blessings/__init__.py b/blessings/__init__.py
index b135e01..bee547e 100644
--- a/blessings/__init__.py
+++ b/blessings/__init__.py
@@ -217,7 +217,10 @@ class Terminal(object):
         return self._height_and_width()[1]
 
     def _height_and_width(self):
-        """Return a tuple of (terminal height, terminal width)."""
+        """Return a tuple of (terminal height, terminal width),
+           using TIOCGWINSZ (Terminal I/O-Control: Get Window Size),
+           falling back to environment variables (LINES, COLUMNS),
+           and 25 x 80 when otherwise unavailable."""
         # tigetnum('lines') and tigetnum('cols') update only if we call
         # setupterm() again.
         for descriptor in self._init_descriptor, sys.__stdout__:
@@ -225,8 +228,13 @@ class Terminal(object):
                 return struct.unpack(
                         'hhhh', ioctl(descriptor, TIOCGWINSZ, '\000' * 8))[0:2]
             except IOError:
+                # when the output stream or init descriptor is not a tty, such
+                # as when when stdout is piped to another program, fe. tee(1),
+                # these ioctls will raise IOError
                 pass
-        return None, None  # Should never get here
+        lines, cols = (int(environ.get('LINES', '24')),
+                       int(environ.get('COLUMNS', '80')))
+        return lines, cols
 
     @contextmanager
     def location(self, x=None, y=None):