I'm writing a wrapper script for a program that optionally accepts input from STDIN. My script needs to process each line of the file, but it also needs to forward STDIN to the program it is wrapping. In minimalist form, this looks something like this:
import subprocess
import sys
for line in sys.stdin:
# Do something with each line
pass
subprocess.call(['cat'])
Note that I'm not actually trying to wrap cat, it just serves as an example to demonstrate whether or not STDIN is being forwarded properly.
With the example above, if I comment out the for-loop, it works properly. But if I run it with the for-loop, nothing gets forwarded because I've already read to the end of STDIN. I can't seek(0) to the start of the file because you can't seek on streams.
One possible solution is to read the entire file into memory:
import subprocess
import sys
lines = sys.stdin.readlines()
for line in lines:
# Do something with each line
pass
p = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
p.communicate(''.join(lines))
which works, but isn't very memory efficient. Can anyone think of a better solution? Perhaps a way to split or copy the stream?
Additional Constraints:
The subprocess can only be called once. So I can't read a line at a time, process it, and forward it to the subprocess.
The solution must work in Python 2.6
Does this work for you?
#!/usr/bin/env python2
import subprocess
import sys
p = subprocess.Popen(['cat'], stdin = subprocess.PIPE)
line = sys.stdin.readline()
####################
# Insert work here #
####################
line = line.upper()
####################
p.communicate(line)
Example:
$ echo "hello world" | ./wrapper.py
HELLO WORLD
Related
I wanted to run a test function called test.counttest() that counts up to 10.
def counttest():
x = 0
for x in range(0,3):
x = x+1
print("Number: "+ str(x))
time.sleep(1)
I want to call just the function from the command line OR from subprocess popen. Not write the function, just call it. Everything I have google keeps bringing me back to how I can write a function from the command line which is NOT what I need.
I need to specifically run a function from subprocess popen so I can get the stdout in a forloop that can then be sent to a flask socket. (This is required)
Main point - How can Call (not write) a function from the command line or from subprocess?
Not this:
python -c 'import whatever then add code'
But something like this:
python "test.counttest()"
or like this:
subprocess.Popen(['python', ".\test.counttest()"],stdout=subprocess.PIPE, bufsize=1,universal_newlines=True)
EDIT:
This is for #Andrew Holmgren. Consider the following script:
def echo(ws):
data = ws.receive()
with subprocess.Popen(['powershell', ".\pingtest.ps1"],stdout=subprocess.PIPE, bufsize=1,universal_newlines=True) as process:
for line in process.stdout:
line = line.rstrip()
print(line)
try:
ws.send(line+ "\n")
except:
pass
this works perfectly for what I need as it:
takes the script's stdout and send's it to the ws.send() function which is a websocket.
However I need this same concept for a function instead. The only way I know how to get the stdout easily is from using subprocess.popen but if there is another way let me know. This is why I am trying to make a hackjob way of running a function through the subprocess module.
The question of Run python function from command line or subprocess popen relates in the fact that if I can get a function to run from subprocess, then I know how to get the stdout for a websocket.
Actually you have really a lot of questions inside this one.
How can I send output of function line-by-line to another one (and/or websocket)? Just avoid writing to stdout and communicate directly. yield (or other generator creation methods) are intended exatly for that.
import time
def counttest():
for i in range(10):
yield f'Item {i}'
time.sleep(1)
def echo(ws):
# data = ws.receive()
for row in counttest():
ws.send(row)
How to call a function func_name defined in file (suppose it's test.py) from command line? Being in directory with test.py, do
$ python -c 'from test import func_name; func_name()'
How to read from sys.stdout? The easiest will be to replace it with io.StringIO and restore thing back later.
from contextlib import redirect_stdout
import io
def echo(ws):
f = io.StringIO()
with redirect_stdout(f):
counttest()
output = f.getvalue()
ws.send(output)
It will return only after call_function(), so you cannot monitor real-time printed items.
Regarding
I need the stdout
I can say, that I'm sure your question is X-Y problem, thus I try to suggest alternatives. Solution you want will also work, but it's awkward. This will exactly run function counttest defined in test.py, capture its output and send it line-by-line to websocket. It will process output immediately when new line arrives. Note -u flag on python call (unbuffered), it's important.
import subprocess
import shlex
def echo(ws):
data = ws.receive()
with subprocess.Popen(shlex.split("python -u -c 'from test import counttest; counttest()'"),
stdout=subprocess.PIPE,
bufsize=1,
universal_newlines=True) as process:
for line in iter(process.stdout.readline, ''):
line = line.rstrip()
if not line:
break
print(line)
try:
ws.send(line + "\n")
except:
pass
I want to read the content of the file which was written to the file by different function
from subprocess import *
import os
def compile():
f=open("reddy.txt","w+")
p=Popen("gcc -c rahul.c ",stdout=f,shell=True,stderr=STDOUT) #i have even tried with with open but it is not working,It is working with r+ but it is appending to file.
f.close()
def run():
p1=Popen("gcc -o r.exe rahul.c",stdout=PIPE,shell=True,stderr=PIPE)
p2=Popen("r.exe",stdout=PIPE,shell=True,stderr=PIPE)
print(p2.stdout.read())
p2.kill()
compile()
f1=open("reddy.txt","w+")
first_char=f1.readline() #unable to read here ….!!!!!!
print(first_char)
#run()
first_char must have first line of file reddy.txt but it is showing null
You are assuming that Popen finishes the process, but it doesn't; Popen will merely start a process - and unless the compilation is extremely fast, it's quite likely that reddy.txt will be empty when you try to read it.
With Python 3.5+ you want subprocess.run().
# Don't import *
from subprocess import run as s_run, PIPE, STDOUT
# Remove unused import
#import os
def compile():
# Use a context manager
with open("reddy.txt", "w+") as f:
# For style points, avoid shell=True
s_run(["gcc", "-c", "rahul.c "], stdout=f, stderr=STDOUT,
# Check that the compilation actually succeeds
check=True)
def run():
compile() # use the function we just defined instead of repeating youself
p2 = s_run(["r.exe"], stdout=PIPE, stderr=PIPE,
# Check that the process succeeds
check = True,
# Decode output from bytes() to str()
universal_newlines=True)
print(p2.stdout)
compile()
# Open file for reading, not writing!
with open("reddy.txt", "r") as f1:
first_char = f1.readline()
print(first_char)
(I adapted the run() function along the same lines, though it's not being used in any of the code you posted.)
first_char is misleadingly named; readline() will read an entire line. If you want just the first byte, try
first_char = f1.read(1)
If you need to be compatible with older Python versions, try check_output or check_call instead of run. If you are on 3.7+ you can use text=True instead of the older and slightly misleadingly named universal_newlines=True.
For more details about the changes I made, maybe see also this.
If you have a look at the documentation on open you can see that when you use w to open a file, it will first truncate that files contents. Meaning there will be no output as you describe.
Since you only want to read the file you should use r in the open statement:
f1 = open("reddy.txt", "r")
I'm writing a python script that reads a stream from stdin and passes this stream to subprocess for further processing. The problem is that python hangs after having processed the input stream.
For example, this toy program sorter.py should read from stdin and pass the stream to subprocess for sorting via Unix sort:
cat dat.txt | ./sorter.py
Here's sorter.py:
#!/usr/bin/env python
import subprocess
import sys
p= subprocess.Popen('sort -', stdin= subprocess.PIPE, shell= True)
for line in sys.stdin:
p.stdin.write(line)
sys.exit()
The stream from cat is correctly sorted but the programs hangs, i.e. sys.exit() is never reached.
I've read quite a few variations on this theme but I can't get it right. Any idea what is missing?
Thank you!
Dario
My guess: sys.exit() is reached but sort continues to run. You should close p.stdin pipe to signal EOF to sort:
#!/usr/bin/env python2
import subprocess
import sys
p = subprocess.Popen('sort', stdin=subprocess.PIPE, bufsize=-1)
with p.stdin:
for line in sys.stdin:
# use line here
p.stdin.write(line)
if p.wait() != 0:
raise Error
Example:
$ < dat.txt ./sorter.py
If you don't need to modify the stdin stream then you don't need to use PIPE here:
#!/usr/bin/env python
import subprocess
subprocess.check_call('sort')
you probably have a problem with buffering -the OS doesnt send the data the moment it arrives to stdin - check this out
https://www.turnkeylinux.org/blog/unix-buffering
I have two scripts which are connected by Unix pipe. The first script writes strings to standard out, and these are consumed by the second script.
Consider the following
# producer.py
import sys
import time
for x in range(10):
sys.stdout.write("thing number %d\n"%x)
sys.stdout.flush()
time.sleep(1)
and
# consumer.py
import sys
for line in sys.stdin:
print line
Now, when I run: python producer.py | python consumer.py, I expect to see a new line of output each second. Instead, I wait 10 seconds, and I suddenly see all of the output at once.
Why can't I iterate over stdin one-item-at-a-time? Why do I have to wait until the producer gives me an EOF before the loop-body starts executing?
Note that I can get to the correct behavior if I change consumer.py to:
# consumer.py
import sys
def stream_stdin():
line = sys.stdin.readline()
while line:
yield line
line = sys.stdin.readline()
for line in stream_stdin():
print line
I'm wondering why I have to explicitly build a generator to stream the items of stdin. Why doesn't this implicitly happen?
According to the python -h help message:
-u Force stdin, stdout and stderr to be totally unbuffered. On systems where it matters, also put stdin, stdout and stderr in
binary mode. Note that there is internal buffering in xread‐
lines(), readlines() and file-object iterators ("for line in
sys.stdin") which is not influenced by this option. To work
around this, you will want to use "sys.stdin.readline()" inside
a "while 1:" loop.
I got a simple python script which should read from stdin.
So if I redirect a stdout of a program to the stdin to my python script.
But the stuff that's logged by my program to the python script will only "reach" the python script when the program which is logging the stuff gets killed.
But actually I want to handle each line which is logged by my program as soon as it is available and not when my program which should actually run 24/7 quits.
So how can I make this happen? How can I make the stdin not wait for CTRL+D or EOF until they handle data?
Example
# accept_stdin.py
import sys
import datetime
for line in sys.stdin:
print datetime.datetime.now().second, line
# print_data.py
import time
print "1 foo"
time.sleep(3)
print "2 bar"
# bash
python print_data.py | python accept_stdin.py
Like all file objects, the sys.stdin iterator reads input in chunks; even if a line of input is ready, the iterator will try to read up to the chunk size or EOF before outputting anything. You can work around this by using the readline method, which doesn't have this behavior:
while True:
line = sys.stdin.readline()
if not line:
# End of input
break
do_whatever_with(line)
You can combine this with the 2-argument form of iter to use a for loop:
for line in iter(sys.stdin.readline, ''):
do_whatever_with(line)
I recommend leaving a comment in your code explaining why you're not using the regular iterator.
It is also an issue with your producer program, i.e. the one you pipe stdout to your python script.
Indeed, as this program only prints and never flushes, the data it prints is kept in the internal program buffers for stdout and not flushed to the system.
Add sys.stdout.flush() call right after you print statement in print_data.py.
You see the data when you quit the program as it automatically flushes on exit.
See this question for explanation,
As said by #user2357112 you need to use:
for line in iter(sys.stdin.readline, ''):
After that you need to start python with the -u flag to flush stdin and stdout immediately.
python -u print_data.py | python -u accept_stdin.py
You can also specify the flag in the shebang.