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Awk in Python's subprocess giving Invalid Expressions "'" error

I am trying to read the filename and filestamp for the most recent files of each of the two naming schemes as seen in the code. I have the following code, roughly:
#!/usr/bin/env python
import string, subprocess, sys, os
mypath = "/path/to/file"
my_cmd = (["ls -lt --full-time " + mypath + "*DAI*.txt",
"ls -lt --full-time " + mypath + "*CA*.txt"]
)
getmostrecent_cmd = "head -n 1"
getcols_cmd = "awk '{ print $6, $7, $9 }'"
for cmd in my_cmd:
p1 = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE)
p2 = subprocess.Popen(getmostrecent_cmd.split(), stdin=p1.stdout, stdout=subprocess.PIPE)
p3 = subprocess.Popen(getcols_cmd.split(), stdin=p2.stdout, stdout=subprocess.PIPE)
output = p3.communicate()[0]
print output
which give me the following error(s):
ls: cannot access /path/to/file/*DAI*.txt: No such file or directory
awk: '{
awk: ^ invalid char ''' in expression
ls: cannot access /path/to/file/*CA*.txt: No such file or directory
awk: '{
awk: ^ invalid char ''' in expression
But:
I can use "ls -lt --full-time /path/to/file/*DAI*.txt" and get a result in the terminal. Why is it causing an issue with the same path?
The awk command, when put in to subprocess directly, works fine; E.g. subprocess.Popen(["awk", ....], stdin=...., stdout=....) worked okay. But now I am getting an issue with the single quote. I tried triple quoting the string and escaping the single-quote.

I can use "ls -lt --full-time /path/to/file/DAI.txt" and get a
result in the terminal. Why is it causing an issue with the same path?
Glob expansion is performed by the shell. By default, shell is not involved in starting a new subprocess via Popen(). To this end you must pass the shell=True argument to it:
p1 = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE, shell=True)
# ^^^^^^^^^^
The awk command, when put in to subprocess directly, works fine; E.g.
subprocess.Popen(["awk", ....], stdin=...., stdout=....) worked okay. But now I am getting an issue with the single quote. I tried
triple quoting the string and escaping the single-quote.
On the shell command line the single quotes in awk '{ print $6, $7, $9 }' are needed to make the string { print $6, $7, $9 } treated as a single argument (as well as to prevent the variable expansion). The single quotes are removed by the shell, and awk only sees the string { print $6, $7, $9 }. Since Popen() by default doesn't involve shell when executing the subprocess command and passes the arguments to the command verbatim, you don't need the single quotes:
subprocess.Popen(["awk", "{ print $6, $7, $9 }"], stdin=...., stdout=....)

Using subprocess to get output

Using the subprocess module how do I get the following command to work?
isql -v -b -d, DSN_NAME "DOMAIN\username" password <<<
"SELECT column_name, data_type
FROM database_name.information_schema.columns
WHERE table_name = 'some_table';"
This command works perfectly when I run it in a bash shell but I can't get it to work when running from within Python. I'm trying to do this from within Python because I need to be able to modify the query and get different result sets back and then process them in Python. I can't use one of the nice Python database connectors for various reasons which leaves me trying to pipe output from isql.
My code currently looks similar to the following:
bash_command = '''
isql -v -b -d, DSN_NAME "DOMAIN\username" password <<<
"SELECT column_name, data_type
FROM database_name.information_schema.columns
WHERE table_name = 'some_table';"
'''
process = subprocess.Popen(bash_command,
shell=True,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
output, error = process.communicate()
However I have tried lots of variations:
Using the entire command as a string, or as a list of strings.
Using check_output vs Popen.
Using communicate() to try and send the query to the isql command or having the query be part of the command string using a heredoc.
Using shell = True or not.
Specifying /bin/bash or using the default /bin/sh.
Lots of different quoting and escaping patterns.
And pretty much every permutation of the above.
In no case do I receive the output of the query that I'm looking for. I'm pretty sure that the command isn't being sent to the shell as is but I can't tell what is being sent to the shell.
I feel like this should be pretty simple, send a command to the shell and get the output back, but I just can't make it work. I can't even see what command is being sent to the shell, even using pdb.

shell=True makes subprocess use /bin/sh by default. <<< "here-string" is a bash-ism; pass executable='/bin/bash':
>>> import subprocess
>>> subprocess.call(u'cat <<< "\u0061"', shell=True)
/bin/sh: 1: Syntax error: redirection unexpected
2
>>> subprocess.call(u'cat <<< "\u0061"', shell=True, executable='/bin/bash')
a
0
You should also use raw-string literals to avoid escaping backslashes: "\\u0061" == r"\u0061" != u"\u0061":
>>> subprocess.call(r'cat <<< "\u0061"', shell=True, executable='/bin/bash')
\u0061
0
Though you don't need shell=True here. You could pass the input as a string using process.communicate(input=input_string):
>>> process = subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
>>> process.communicate(br"\u0061")
('\\u0061', None)
The result could look like:
#!/usr/bin/env python
import shlex
from subprocess import Popen, PIPE
cmd = shlex.split(r'isql -v -b -d, DSN_NAME "DOMAIN\username" password')
process = Popen(cmd, stdin=PIPE, stdout=PIPE, stderr=PIPE)
output, errors = process.communicate(
b"SELECT column_name, data_type "
b"FROM database_name.information_schema.columns "
b"WHERE table_name = 'some_table';")

Try giving this a shot:
import shlex
from subprocess import Popen, PIPE, STDOUT
sql_statement = '''"SELECT column_name, data_type
FROM database_name.information_schema.columns
WHERE table_name = 'some_table';"'''
isqlcommand = 'isql -v -b -d, DSN_NAME "DOMAIN\username" password'
isqlcommand_args = shlex.split(isqlcommand)
process = Popen(isqlcommand_args, stdin=PIPE, stdout=PIPE, stderr=STDOUT)
output = process.communicate(input=sql_statement)[0]
print output
The idea here is to separate the here-string redirection from the isql command execution. This example will pipe the here-string into the stdin of process via process.communicate(). I'm also using shlex.split() to tokenize the command and its arguments.
Edit: Removed Shell=True after reviewing comment from J.F. Sebastian

Escaping in Python subprocess for sed command

I am using Python to do sed delete / replace with the help of the subprocess. Somehow I am not getting the number of escapes correct. Here is my code:
from subprocess import call
remover = ["sed", "-i", "'1d'", "file"]
call(remover)
removeq = ["sed", "-i", "'s/\"//g'", "file"]
call(removeq)
Both of these tasks produce the error message:
sed: -e expression #1, char 1: unknown command: `''
How many times does the ' sign need to be escaped in the first list and how many times does the " have to be escaped in the last one? I tried once, twice and three times, but to no avail. There are also no hidden characters that can potentially mess something up.
Any suggestions?

Fix the quoting mechanism:
sed -i 's/\"//g' file
Should be just:
sed -i 's/"//g' file
You can also take adventage of shlex library.
Example from interpreter:
>>> import shlex
>>> cmd = "sed -i '1d' file"
>>> shlex.split(cmd)
['sed', '-i', '1d', 'file']
>>> cmd = """sed -i 's/"//g' file"""
>>> shlex.split(cmd)
['sed', '-i', 's/"//g', 'file']

Large command in Python using subprocess

How could I run this code using subprocess module?
commands.getoutput('sudo blkid | grep 'uuid' | cut -d " " -f 1 | tr -d ":"')
I've tried this but it doesn't work at all
out_1 = subprocess.Popen(('sudo', 'blkid'), stdout=subprocess.PIPE)
out_2 = subprocess.Popen(('grep', 'uuid'), stdin=out_1.stdout, stdout=subprocess.PIPE)
out_3 = subprocess.Popen(('cut', '-d', '" "', '-f', '1'), stdin=out_2.stdout, stdout=subprocess.PIPE)
main_command = subprocess.check_output(('tr', '-d', '":"'), stdin=out_3.stdout)
main_command
Error: cut: the delimiter must be a single character

from subprocess import check_output, STDOUT
shell_command = '''sudo blkid | grep 'uuid' | cut -d " " -f 1 | tr -d ":"'''
output = check_output(shell_command, shell=True, stderr=STDOUT,
universal_newlines=True).rstrip('\n')
btw, it returns nothing on my system unless grep -i is used. In the latter case it returns devices. If it is your intent then you could use different command:
from subprocess import check_output
devices = check_output(['sudo', 'blkid', '-odevice']).split()
I'm trying not to use shell=True
It is ok to use shell=True if you control the command i.e., if you don't use user input to construct the command. Consider the shell command as a special language that allows you to express your intent concisely (like regex for string processing). It is more readable then several lines of code that do not use shell:
from subprocess import Popen, PIPE
blkid = Popen(['sudo', 'blkid'], stdout=PIPE)
grep = Popen(['grep', 'uuid'], stdin=blkid.stdout, stdout=PIPE)
blkid.stdout.close() # allow blkid to receive SIGPIPE if grep exits
cut = Popen(['cut', '-d', ' ', '-f', '1'], stdin=grep.stdout, stdout=PIPE)
grep.stdout.close()
tr = Popen(['tr', '-d', ':'], stdin=cut.stdout, stdout=PIPE,
universal_newlines=True)
cut.stdout.close()
output = tr.communicate()[0].rstrip('\n')
pipestatus = [cmd.wait() for cmd in [blkid, grep, cut, tr]]
Note: there are no quotes inside quotes here (no '" "', '":"'). Also unlike the previous command and commands.getoutput(), it doesn't capture stderr.
plumbum provides some syntax sugar:
from plumbum.cmd import sudo, grep, cut, tr
pipeline = sudo['blkid'] | grep['uuid'] | cut['-d', ' ', '-f', '1'] | tr['-d', ':']
output = pipeline().rstrip('\n') # execute
See How do I use subprocess.Popen to connect multiple processes by pipes?

pass your command as one string like this:
main_command = subprocess.check_output('tr -d ":"', stdin=out_3.stdout)
if you have multiple commands and if you want to execute one by one, pass them as list:
main_command = subprocess.check_output([comand1, command2, etc..], shell=True)

Popen.communicate escapes a string I send to stdin

I am trying to spawn a process using Popen and send it a particular string to its stdin.
I have:
pipe = subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE)
pipe.communicate( my_stdin_str.encode(encoding='ascii') )
pipe.stdin.close()
However, the second line actually escapes the whitespace in my_stdin_str. For example, if I have:
my_stdin_str="This is a string"
The process will see:
This\ is\ a\ string
How can I prevent this behaviour?

I can't reproduce it on Ubuntu:
from subprocess import Popen, PIPE
shell_cmd = "perl -pE's/.\K/-/g'"
p = Popen(shell_cmd, shell=True, stdin=PIPE)
p.communicate("This $PATH is a string".encode('ascii'))
In this case shell=True is unnecessary:
from subprocess import Popen, PIPE
cmd = ["perl", "-pE" , "s/.\K/-/g"]
p = Popen(cmd, stdin=PIPE)
p.communicate("This $PATH is a string".encode('ascii'))
Both produce the same output:
T-h-i-s- -$-P-A-T-H- -i-s- -a- -s-t-r-i-n-g-

Unless you know you need it for some reason, don't run with "shell=True" in general (which, without testing, sounds like what's going on here).