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Echo filtering

When you type something in a console or in an interactive interpreter, what you type is echoed so you can see what you are typing.

byexample disables this so it can capture the output of an example without having the typed example mixed with it.

However not all the interpreters honors this and they may leave turned on the echo anyway.

For example this shell example uses stty to re-enable the echoing:

$ echo "normal output"
normal output

$ stty echo

$ echo "normal output"
echo "normal output"
normal output

Having to expect what you typed is ugly and confusing for your readers.

byexample has the experimental ability to suppress the echoing with +force-echo-filtering:

$ echo "normal output"      # byexample: +force-echo-filtering
normal output

Limitations and restrictions

Forcing a filtering comes with some restrictions:

  • the example will use a full terminal emulation, in other words +force-echo-filtering implies +term=ansi.
  • due the emulation, your output will be limited by the geometry of the emulated terminal; you may have to set this too at the begin of your document.
  • the echo must exist otherwise the filter may filter part of your output.
  • it is an experimental feature.

If possible, try to disable the echo from the interpreter itself and relay on +force-echo-filtering as a last resort.

For example, in python you can use termios to control the echoing:

>>> import sys
>>> from termios import tcgetattr, tcsetattr, TCSANOW, ECHO

>>> def set_echo_mode(enable):
...     fd = sys.stdin.fileno()
...     attrs = tcgetattr(fd)
...     if enable:
...         attrs[3] |= ECHO
...     else:
...         attrs[3] &= ~ECHO
...     tcsetattr(fd, TCSANOW, attrs)

>>> set_echo_mode(True)

>>> print("normal output")
print("normal output")
normal output

>>> set_echo_mode(False)

>>> print("normal output")
normal output