python3 pexpect spawn object line reference gets skipped - python-3.x

Question

Python version: 3.8.0
pexpect Version: 4.8.0
def runCmd( self, cmd, thisTimeout = None ):
output = ""
if not thisTimeout:
thisTimeout = self.conn.timeout
try:
print("debug: %s" % cmd)
self.conn.sendline(cmd)
print( "before: %s " % self.conn.before.decode() )
index = self.conn.expect( self.expList, timeout = thisTimeout )
output += self.conn.before.decode()
print( "after: %s " % self.conn.after.decode() )
print( "before after: %s" % self.conn.before.decode() )
except Exception as e:
#expect exception thrown
print( "Error running command %s" % cmd )
print( e )
output = "Error: %s" % str(self.conn)
print("yo man %s" % self.conn.before.decode() )
output = output.replace(cmd, "").strip()
print("this has to print %s " % output)
return output
This function executes the cmd through the pexpect interface and returns the output.
Version of Python/pxpect that worked:
Python version: 3.6.9
pexpect version: 4.2.1
After an update of the python script to run on Python 3.8.0/pexpect 4.8.0, the first command sent to pexpect sometimes returns empty output. The reason is when the variable self.conn.before.decode() gets referenced, the python code does not get executed or ineffective.
An example output from described situation:
debug: cat /etc/hostname
before:
after: ubuntu#ip-172-31-1-219:~$
this has to print
An expected behavior:
debug: cat /etc/hostname
after: ubuntu#ip-172-31-1-219:~$
before after: cat /etc/hostname
ip-172-31-1-219
yo man cat /etc/hostname
ip-172-31-1-219
this has to print ip-172-31-1-219
But this time, the line before: gets skipped.
What is going on here?!
Downgrade is not possible as async(pexpect(<=4.2.1) used async as function/variable signature) becomes a keyword.
Update:
The lines are getting executed but it's printing out after I print it as byte string.
before after: b' \r\x1b[K\x1b]0;ubuntu#ip-172-31-1-219: ~\x07'
Where the correct one is printing out
before after: b' cat /etc/hostname\r\nip-172-31-1-219
\r\n\x1b]0;ubuntu#ip-172-31-1-219: ~\x07'

Answer 1

The reason the before and before after lines get skipped is that they contain the carriage return character \r and the escape sequence \x1b[K.
The carriage return is used to move the cursor to the start of the line. If there are characters after it in the string to be written, they get printed from the position of the cursor onward replacing existing printed characters.
The ANSI Control sequence \x1b[K erases the line from the position of the cursor to the end of the line. This clears the already printed strings in your particular case.