Understanding Popen communicate

Question

I have a script named 1st py which creates a REPL  read-eval-print-loop    print  Something to print  while True      r   raw input       if r     n           print  exiting          break     else          print  continuing    I then launched 1st py with the following code   p   subprocess Popen   python   1st py    stdin PIPE  stdout PIPE    And then tried this   print p communicate   0    It failed  providing this traceback   Traceback  most recent call last     File  1st py   line 3  in  lt module gt      r   raw input   EOFError  EOF when reading a line   Can you explain what is happening here please  When I use p stdout read    it hangs forever

User · Answer

Your second bit of code starts the first bit of code as a subprocess with piped input and output. It then closes its input and tries to read its output.

The first bit of code tries to read from standard input, but the process that started it closed its standard input, so it immediately reaches an end-of-file, which Python turns into an exception.

User · Answer

communicate   writes input  there is no input in this case so it just closes subprocess  stdin to indicate to the subprocess that there is no more input   reads all output  and waits for the subprocess to exit   The exception EOFError is raised in the child process by raw input    it expected data but got EOF  no data     p stdout read   hangs forever because it tries to read all output from the child at the same time as the child waits for input  raw input    that causes a deadlock   To avoid the deadlock you need to read write asynchronously  e g   by using threads or select  or to know exactly when and how much to read write  for example   from subprocess import PIPE  Popen  p   Popen   python    -u    1st py    stdin PIPE  stdout PIPE  bufsize 1  print p stdout readline      read the first line for i in range 10     repeat several times to show that it works     print  gt  gt p stdin  i   write input     p stdin flush     not necessary in this case     print p stdout readline      read output  print p communicate  n n   0     signal the child to exit                                   read the rest of the output                                    wait for the child to exit   Note  it is a very fragile code if read write are not in sync  it deadlocks   Beware of block-buffering issue  here it is solved by using  -u  flag that turns off buffering for stdin  stdout in the child    bufsize 1 makes the pipes line-buffered on the parent side

User · Answer

Do not use communicate input      It writes input to the process  closes its stdin and then reads all output   Do it like this   p subprocess Popen   python   1st py   stdin PIPE stdout PIPE     get output from process  Something to print  one line output   p stdout readline      write  a line n  to the process p stdin write  a line n      get output from process  not time to break  one line output   p stdout readline       write  n n  to that process for if r   n   p stdin write  n n       read the last output from the process   Exiting  one line output   p stdout readline     What you would do to remove the error   all the process will tell you   p communicate  all you will ever say to this process nn n   0    But since communicate closes the stdout and stdin and stderr  you can not read or write after you called communicate

[python] Understanding Popen.communicate

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