TextIOTrap

Simple class for trapping / capturing Python Text IO streams like from subprocess.popen, pexpect, sys.stderr and others; enabling the capture output of or dropping streams with cross platform DEVNULL helper.

Installation

python3 -m pip install tiotrap

Usage

This tool contains one class TextIOTrap and a helper DEVNULL.

Examples

(Ex1) Use TextIOTrap to capture stdout of a chatty process using store option:

_stdout_bk = sys.stdout # Store original stdout
ttrap = tiotrap.TextIOTrap(store=True)

try:
    sys.stdout = ttrap # Map stdout to tiotrap
    print("TEST1")
    # call some chatty functions()
    print("TEST2")

finally:
    sys.stdout = _stdout_bk # Restore stdout

Output of print:

captured logs:
TEST1
<chatty outputs here>
TEST2

(Ex2) Use TextIOTrap to capture stdout using write_handler option:

aTrap = []
_stdout_bk = sys.stdout
try:
    sys.stdout = tiotrap.TextIOTrap(write_handler=lambda s: aTrap.append(s))
    print("TEST1")
    print("TEST2")

finally:
    sys.stdout = _stdout_bk
# print adds extra \n end so remove with rstrip()
print(f"aTrap:\n{''.join(aTrap).rstrip()}\n~end~\n")

Output of print:

aTrappedStdout = ['TEST1', 'TEST2']

You can substitute lambda with a function or method call to handle writes with your own code.

(Ex3) Use TextIOTrap grab output pexpect call :

ttrap = tiotrap.TextIOTrap(store=True)

p = pexpect.spawn('ls -la')
p.logfile = ttrap
p.expect(pexpect.EOF)

print(f"`ls -la` cmd output:\n{ttrap.entries()}\n~")

Output of print:

ls output:
<full directory listing here of cwd>

Other uses of TextIOTrap:

• Output the stdout of a subprocess.popen call in real time to a secondary log file
• ...

(Ex4) Use TextIOTrap grab output pexpect call :

ttrap = tiotrap.TextIOTrap(store=True)

p = pexpect.spawn('ls -la')
p.logfile = ttrap
p.expect(pexpect.EOF)

print("ls -la` cmd output (as was written):")
for write in ttrap:
    print(write)

Output: Similar to Ex4

(Ex5) Use DEVNULL to drop all output of a TextIO Stream

_stdout_bk = sys.stdout

try:
    sys.stdout = tiotrap.DEVNULL
    print("THIS WILL NOT PRINT")

finally:
    sys.stdout = _stdout_bk

print("THIS WILL PRINT")

This DEVNULL is very simple implementation and is fully cross platform unlike someother DEVNULL implementations.

Note: TextIOTrap has been set up to be compatible with the standard methods for a Text IO streams. I'll be glad to update if any edge cases are discovered.