Improve handling of situation when stdout is not a terminal.

Print warning and render anyway instead of failing with an assertion.

Author: jonathanslenders

prompt_toolkit/output/vt100.py

@@ -18,6 +18,7 @@
 import array
 import errno
 import six
+import sys
 
 __all__ = [
  'Vt100_Output',
@@ -380,6 +381,9 @@ class Vt100_Output(Output):
  :param write_binary: Encode the output before writing it. If `True` (the
  default), the `stdout` object is supposed to expose an `encoding` attribute.
  """
+ _fds_not_a_terminal = set() # For the error messages. Only display "Output
+ # is not a terminal" once per file descriptor.
+
  def __init__(self, stdout, get_size, term=None, write_binary=True):
  assert callable(get_size)
  assert term is None or isinstance(term, six.text_type)
@@ -409,11 +413,23 @@ def from_pty(cls, stdout, term=None):
  (This will take the dimensions by reading the pseudo
  terminal attributes.)
  """
- assert stdout.isatty()
+ # Normally, this requires a real TTY device, but people instantiate
+ # this class often during unit tests as well. For convenience, we print
+ # an error message, use standard dimensions, and go on.
+ isatty = stdout.isatty()
+ fd = stdout.fileno()
+
+ if not isatty and fd not in cls._fds_not_a_terminal:
+ msg = 'Warning: Output is not to a terminal (fd=%r).\n'
+ sys.stderr.write(msg % fd)
+ cls._fds_not_a_terminal.add(fd)
 
  def get_size():
- rows, columns = _get_size(stdout.fileno())
- return Size(rows=rows, columns=columns)
+ if isatty:
+ rows, columns = _get_size(stdout.fileno())
+ return Size(rows=rows, columns=columns)
+ else:
+ return Size(rows=24, columns=80)
 
  return cls(stdout, get_size, term=term)