Python Basic Pipes

Python provides two main avenues of parallel processing. One avenue is to use multithreading where a program itself multitasks, while the other approach is to have a program relaunch itself as a separate program in a new process. One approach is not necessarily better than the other approach but instead, should be throught of as tools for different use cases. Threads have low overhead and share a program’s memory space, which allows for easy communication between threads. Processes operate as if we launched a new copy of the program from our operating system and allow programs to spread themselves out over an operating system or even a network.

However, processes do not share a global memory space, which means they need a way to communicate with one another. One approach to interprocess communication (IPC) is to use pipes. This post shows an example of IPC using pipes taken from Programming Python: Powerful Object-Oriented Programming. I have added my own comments to the code for clarity.

Code

import os, time


# Function called by child processes
def child(pipeout):
    zzz = 0
    while True:
        time.sleep(zzz)

        # We have to encode our string to binary to use
        # with pipes
        msg = ('Spam {}'.format(zzz)).encode()

        # Send the data back to the parent process
        os.write(pipeout, msg)
        zzz = (zzz + 1) % 5


def parent():
    # Creates our pipes. The pipeout gets passed to the child
    # process while parent keeps pipein
    pipein, pipeout = os.pipe()

    if os.fork() == 0:
        # We are now in the child process so call child and supply
        # it with pipeout so that it can send information back to
        # the parent.
        child(pipeout)
    else:
        # This is the parent process
        while True:
            # Read data from the child process
            # This call blocks until there is data
            line = os.read(pipein, 32)

            # Print to the console
            print('Parent {} got [{}] as {}'.format(os.getpid(), line, time.time()))


if __name__ == '__main__':
    parent()

Explanation

We have two functions in the program named child() and parent(). The child() function is intended to run in child processes while parent() contains the main program. Parent() is defined on lines 19-37. The function begins by calling os.pipe() on line 22 which returns a tuple containing two ends of a single pipe. Pipes are unidirectional and thus pipein is used by the parent to read data that comes from the child process. The child process uses pipeout to send data to the parent.

The program forks into two different processes on line 24. The program is in the child process when os.fork() returns zero. Line 28 calls the child() function and passes pipeout to the child function so that the child process can send data back to the parent. The child process enters an infinite loop on line 7. On line 12, a msg variable is created that contains a String variable. Pipe send binary data, so we have to call encode() on the String to convert it to a binary string. Then on line 15, we send the msg varaiable back to the parent using os.write and supplying pipeout and msg to that function.

The parent process continues on line 31. It attempts to read data from the child process on line 34 using os.read. Notice that os.read requires a pipein variable and the size of binary data to read (32 bytes in this program). If the pipe contains data, os.read returns immedialy and stores the value in the line variable. Otherwise, os.read blocks the program until the pipe has data. The parent process prints the data on line 37.

References

Lutz, Mark. Programming Python. Beijing, OReilly, 2013.

Advertisements

Python Fork() Exit Status

Most computer programs return an exit code to the operating system’s shell. Shell scripting tools can use the exit status of a program to indicate if the program exited normally or abnormally. In either case, the shell script can react depending on the outcome of the child process.

Python programs can use os.fork() to create child processes. Since the child process is a new instance of the program, it can be useful in some cases to inspect if the child process exited normally or not. For example, a GUI program may spawn a child process and notify the user if the operation completed successfully or not.

This post shows an example program taken from Programming Python: Powerful Object-Oriented Programming that demonstrates how a parent process can inspect a child process’ exit code. I added comments to help explain the workings of the program.

Code

import os

exitstat = 0


# Function that is executed after os.fork() that runs in a new process
def child():
    global exitstat
    exitstat += 1
    print('Hello from child', os.getpid(), exitstat)
    
    # End this process using os._exit() and pass a status code back to the shell
    os._exit(exitstat)


# This is the parent process code
def parent():
    while True:
        # Fork this program into a child process
        newpid = os.fork()
        
        # newpid is 0 if we are in the child process
        if newpid == 0:
            # Call child()
            child()
            
        # otherwise, we are still in the parent process
        else:
            # os.wait() returns the pid and status and status code
            # On unix systems, status code is stored in status and has to
            # be bit-shifted
            pid, status = os.wait()
            print('Parent got', pid, status, (status >> 8))
            if input() == 'q':
                break


if __name__ == '__main__':
    parent()

Explanation

This program is pretty basic. We have two functions, parent() and child(). When the program starts on line 38, it calls parent() to enter the parent() function. The parent() function enters and infinite loop that forks this program on line 20. The result of os.fork() is stored in the newpid variable.

Our program is executing in the child process when newpid is zero. If that case, we call our child() function. The child() function prints a message to the console on line 10 and then exits by calling os._exit() on line 13. We pass the exitstat variable to os._exit() whose value becomes the exit code for this process.

The parent process continues in the meantime. On line 32, we use os.wait() to return the pid and status of the child process. The status variable also containes the exitstat value passed to os._exit() in the child process, but to get this code, we have to perform a bit shift operation by eight bits. The following line prints the pid, status, and the child process’ exit code to the console. When the user presses ‘q’, the parent process ends.

References

Lutz, Mark. Programming Python. Beijing, OReilly, 2013.

Use Python to Launch Another Program

Many Python programs operate as wrappers to other programs. For example, there are a wide variety of Python programs that work as a GUI wrapper for CLI programs. This tutorial uses an example from Programming Python: Powerful Object-Oriented Programming to demonstrate how a Python program can be used to launch another program.

Code

Parent Process

This code represents a parent process that is capable of launching a child process. I added my own comments to help further explain the code.

import os

# Track the number of spawned children
spawned_children = 0

# Enter an infinite loop
while True:
    
    # Increment the number of children
    spawned_children += 1
    
    # Now launch a new process
    # If we are still in the parent process, pid is non-zero
    # otherwise, pid is zero and indicates that we are in the
    # child process
    pid = os.fork()
    
    # Test if we are in the child process
    if pid == 0:
        
        # Now, since we are in the new process, we use
        # os.execlp to launch a child script.
        os.execlp('python', 'python', 'child.py', str(spawned_children))
        assert False, 'error starting program'
    else:
        # If we are here, then we are still in the parent process
        print('Child is', pid)
        if input() == 'q': break

Child Process

This is a simple script that simply represents a sample child process.

import os, sys

# Parent passes the child process number
# as a command line argument
print('Hello from child', os.getpid(), sys.argv[1])

Explanation

We have two core concepts used in this program. The first concept is forking the parent process into a child process. The other concept is actually launching a new program.

Forking

Forking is the process of launching a copy of the running program. The program that launched a copy of itself is called the parent process, while the copy of the program is called the child process. In theory, a parent and its children can spawn as many children as the underlying operating system allows.

The example program spawns a child process on line 16 and assigns the result to the variable called pid. When os.fork() returns, pid is zero if we are running in the child process or non-zero if we are still in the parent. The program uses an if – else statement (lines 19 and 25 respectively) to perform different operations depending if we are in the parent or child process.

The parent process executes lines 27 and 28. It simply prints out the child’s process id and then asks the user if they wish to continue the program. The user can enter ‘q’ to exit the parent process.

The child process executes code on lines 23 and 24. Line 23 uses os.execlp to start the companion child script and passes the number of spawned children as a command line argument. The child script prints its pid and the number of child processes and then exits.

Launching Programs

The Python os module provides a variety of ways to launch other programs via a Python script. This example makes use of os.execlp which looks for an executable on the operating system’s path. The first argument is the name of the program followed by any number of command line arguments.

The os.execlp is not supposed to return to the caller because if successful, the program is replaced by the new program. This is why it’s necessary to call os.fork() prior to using os.execlp (unless you wish your program to simply end with the new process). When the example script calls os.execlp, the child process created by os.fork is replaced by the new program launched by os.execlp.

Line 24 handles the case when os.execlp fails. Should os.execlp fail, control is returned back to the caller. In most cases, a develop should notify the user that launching the new program has failed. The example script accomplishes this by using an assert False statement with an error message.

References

Lutz, Mark. Programming Python. Beijing, OReilly, 2013.

Python Basic Forking

Many programs need to execute tasks simultaneously and Python provides us with a few different mechanisms for concurrent programming. One of those mechanisms is called forking, where a call is made to the underlying operating system to create a working copy of a program that’s already running. The program that created the new process is called the parent process, while the processes that are created by the parent are called the child process.

This post shows the most basic form of creating processes in Python and helps serve as a foundation to understanding forking. The example is derived from Programming Python: Powerful Object-Oriented Programming, and I added my own comments to help better explain the program.

import os


# This is a function called by the child process
def child():
    # Use os.getpid() to get the pid for this process
    print('Hello from child', os.getpid())

    # force the child process to exit right away
    # or the child process will return to the infinite loop in parent()
    os._exit(0)


def parent():
    while True:
        # Attempt to fork this program into a new process. When forking is complete
        # newpid will be non-zero for the original process, but it wil be
        # 0 in the child process
        newpid = os.fork()

        # The program now goes in two different directions at the same time
        # When newpid is 0, we call child() and the child process exits
        if newpid == 0:  # Test if this is a child process
            child()
        else:
            # If are here, then we are still in the parent process
            # We print the pid of the parent and the child process (newpid)
            print('Hello from parent', os.getpid(), newpid)
        if input() == 'q':
            break


if __name__ == '__main__':
    parent()

When run on my machine, the program shows the following output

Hello from parent 87800 87802
Hello from child 87802
k
Hello from parent 87800 87803
Hello from child 87803
k
Hello from parent 87800 87804
Hello from child 87804
k
Hello from parent 87800 87805
Hello from child 87805
q

Explanation

The hardest part to grasp about this program is that when os.fork() is called on line 19, the program actually launches a copy of itself. The operating system creates the new process and that new process gets a copy of all variables in memory and execution of the new and old programs continue after line 19. (Note: The OS may not exactly copy the parent process, but functionally speaking, the child process can be considered to be a copy of the parent).

The os.fork() function returns a number called a PID (process ID). We can test the pid to see if we are running in the parent or child process. When we are in the child process, the value returned by os.fork() is zero. So on line 23, we test for 0 and if newpid is zero, we call the child() function.

The alternative case is that we are still running in the parent process (bearing in mind, that the child process is also running at this point in time as well). If we are still in the parent process os.fork() returns a non-zero value. In that case, we use the else block to print the parent and child PID.

The parent process continues to loop until the user enters q to quit. Each time the loop iterates, a new child process is created by the parent. The parent prints its own PID (using os.getpid()) and the pid of the child on line 28.

The child process also uses os.getpid() to get its own PID. It prints its own PID on line 7 and then on line 11, we use os._exit(0) to force the child process to shut down. This is a critical step for this program! If we were to omit the call to os._exit on line 11, the child process would return to the parent function and enter the same infinite loop the parent is using.

Conclusion

This is the most basic example of creating child processes using Python. Keep in mind that processes do not share memory (unlike threads). In real world programs, processes often need to sycnchronize data from one process to another process using tools such as network sockets, databases, or files. When a child process is spawned it gets a copy of the memory of the parent process, but then functions as an independent program.

Source

Lutz, Mark. Programming Python. Beijing, OReilly, 2013

Python Environmental Variables

Every operating system allows system adminstrators and users to set and use environmental variables. Such variables can be highly useful when writing shell scripts that automate commands and executes tasks on the system. It’s highly feasible to imagine situations where build toolchains may need to know about the location of programs such as compilers, linkers, etc.

Python’s os module has an environ object. It acts like a dictionary and allows scripts to read and write to enviornmental variables. Here are two example scripts found in Programming Python: Powerful Object-Oriented Programming that demonstrate using environmental varaibles in Python. I added comments to help explain the programs.

setenv.py

import os

# First print the value associated with the USER
# environmental variable
print('setenv...', end=' ')
print(os.environ['USER'])

# Now overwrite the value stored in USER. It will
# propagate through the system
os.environ['USER'] = 'Brian'

# Run the echoenv script to prove the environmental
# variable has been updated
os.system('python echoenv.py')

# Repeat
os.environ['USER'] = 'Arthur'
os.system('python echoenv.py')

# This time, we ask the user for a name
os.environ['USER'] = input('?')

# Now we are going to run the echoevn script, but connect
# it's output to this process and print
print(os.popen('python echoenv.py').read())

echoenv.py

import os
print('echoenv...')

# Just print the value for USER found in os.environ
print('Hello, ', os.environ['USER'])

Detailed Explanation

Although the main purpose of this script is to demonstrate how to use environmental variables in Python. However, we are going to use the os.system and os.popopen functions to execute our helper echoenv script to help prove that changes to the environmental variables propogate throughout the system.

Line 6 is the first call to os.environ. We look up the USER value in this environ object which prints out the current user to the console. On line 10, we overwrite the value in USER with “Brian”. Now Brian is the user. Line 14 proves the change by calling os.system and executing echoenv.py. That script will print “Brain” in the console.

Lines 17 and 18 are a repeat of lines 10 and 14. Line 21 is only different in the sense that we ask the user for a value this time. Line 25 uses the os.popopen command to execute echoenv.py. This function returns an object that gives us a handle into the sister process. Rather than having echoenv print to the console directly this time, we use the read() method to print the output in the setenv process.

Example Output

Here is the output when run on my machine.

Patricks-MacBook-Pro:Environment stonesoup$ python setenv.py
setenv... stonesoup
echoenv...
('Hello, ', 'Brian')
echoenv...
('Hello, ', 'Arthur')
?Bob Belcher
echoenv...
('Hello, ', 'Bob Belcher')