I'm trying to make a program which takes an executable name as an argument, runs the executable and reports the inputs and outputs for that run. For example consider a child program named "circle". The following would be desired run for my program:
$ python3 capture_io.py ./circle
Enter radius of circle: 10
Area: 314.158997
[('output', 'Enter radius of circle: '), ('input', '10\n'), ('output', 'Area: 314.158997\n')]
I decided to use pexpect module for this job. It has a method called interact which lets the user interact with the child program as seen above. It also takes 2 optional parameters: output_filter and input_filter. From the documentation:
The
output_filterwill be passed all the output from the child process. Theinput_filterwill be passed all the keyboard input from the user.
So this is the code I wrote:
import sys
import pexpect
_stdios = []
def read(data):
_stdios.append(("output", data.decode("utf8")))
return data
def write(data):
_stdios.append(("input", data.decode("utf8")))
return data
def capture_io(argv):
_stdios.clear()
child = pexpect.spawn(argv)
child.interact(input_filter=write, output_filter=read)
child.wait()
return _stdios
if __name__ == '__main__':
stdios_of_child = capture_io(sys.argv[1:])
print(stdios_of_child)
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[]) {
float radius, area;
printf("Enter radius of circle: ");
scanf("%f", &radius);
if (radius < 0) {
fprintf(stderr, "Negative radius values are not allowed.\n");
exit(1);
}
area = 3.14159 * radius * radius;
printf("Area: %f\n", area);
return 0;
}
Which produces the following output:
$ python3 capture_io.py ./circle
Enter radius of circle: 10
Area: 314.158997
[('output', 'Enter radius of circle: '), ('input', '1'), ('output', '1'), ('input', '0'), ('output', '0'), ('input', '\r'), ('output', '\r\n'), ('output', 'Area: 314.158997\r\n')]
As you can observe from the output, input is processed character by character and also echoed back as output which creates such a mess. Is it possible to change this behaviour so that my input_filter will run only when Enter is pressed?
Or more generally, what would be the best way to achieve my goal (with or without pexpect)?
Is it possible to change this behaviour so that my
input_filterwill run only whenEnteris pressed?
Yes, you can do it by inheriting from pexpect.spawn and overwriting the interact method. I will come to that soon.
As VPfB pointed out in their answer, you can't use a pipe and I think it's worth to mentioning that this issue is also addressed in the pexpect's documentation.
You said that:
... input is processed character by character and also echoed back as output ...
If you examine the source code of the interact you can see this line:
tty.setraw(self.STDIN_FILENO)
This will set your terminal to raw mode:
input is available character by character, ..., and all special processing of terminal input and output characters is disabled.
That is why your input_filter function is running for every key press and it sees backspace or other special characters. If you could comment out this line, you would see something like this when you run your program:
$ python3 test.py ./circle
Enter radius of circle: 10
10
Area: 314.158997
[('output', 'Enter radius of circle: '), ('input', '10\n'), ('output', '10\r\n'), ('output', 'Area: 314.158997\r\n')]
And this would also let you edit the input (i. e. 12[Backspace]0 would give you same result). But as you can see, it still echoes the input. This can be disabled by setting a simple flag for child's terminal:
mode = tty.tcgetattr(self.child_fd)
mode[3] &= ~termios.ECHO
tty.tcsetattr(self.child_fd, termios.TCSANOW, mode)
Running with the latest changes:
$ python3 test.py ./circle
Enter radius of circle: 10
Area: 314.158997
[('output', 'Enter radius of circle: '), ('input', '10\n'), ('output', 'Area: 314.158997\r\n')]
Bingo! Now you can inherit from pexpect.spawn and override interact method with these changes or implement the same thing using the builtin pty module of Python:
pty:
import os
import pty
import sys
import termios
import tty
_stdios = []
def _read(fd):
data = os.read(fd, 1024)
_stdios.append(("output", data.decode("utf8")))
return data
def _stdin_read(fd):
data = os.read(fd, 1024)
_stdios.append(("input", data.decode("utf8")))
return data
def _spawn(argv):
pid, master_fd = pty.fork()
if pid == pty.CHILD:
os.execlp(argv[0], *argv)
mode = tty.tcgetattr(master_fd)
mode[3] &= ~termios.ECHO
tty.tcsetattr(master_fd, termios.TCSANOW, mode)
try:
pty._copy(master_fd, _read, _stdin_read)
except OSError:
pass
os.close(master_fd)
return os.waitpid(pid, 0)[1]
def capture_io_and_return_code(argv):
_stdios.clear()
return_code = _spawn(argv)
return _stdios, return_code >> 8
if __name__ == '__main__':
stdios, ret = capture_io_and_return_code(sys.argv[1:])
print(stdios)
with pexpect:
import sys
import termios
import tty
import pexpect
_stdios = []
def read(data):
_stdios.append(("output", data.decode("utf8")))
return data
def write(data):
_stdios.append(("input", data.decode("utf8")))
return data
class CustomSpawn(pexpect.spawn):
def interact(self, escape_character=chr(29),
input_filter=None, output_filter=None):
self.write_to_stdout(self.buffer)
self.stdout.flush()
self._buffer = self.buffer_type()
mode = tty.tcgetattr(self.child_fd)
mode[3] &= ~termios.ECHO
tty.tcsetattr(self.child_fd, termios.TCSANOW, mode)
if escape_character is not None and pexpect.PY3:
escape_character = escape_character.encode('latin-1')
self._spawn__interact_copy(escape_character, input_filter, output_filter)
def capture_io_and_return_code(argv):
_stdios.clear()
child = CustomSpawn(argv)
child.interact(input_filter=write, output_filter=read)
child.wait()
return _stdios, child.status >> 8
if __name__ == '__main__':
stdios, ret = capture_io_and_return_code(sys.argv[1:])
print(stdios)