Basic I/O

Stdout and stderr

Stdout and stderr in your running program will be sent back to the stdout and stderr of your lager python process.

stdio.py

import sys
print('This is stdout')
print('This is stderr', file=sys.stderr)

Save this as stdio.py. Then, we'll run it and redirect the output streams to files: lager python stdio.py > stdout 2> stderr

➜  lager python stdio.py > stdout 2> stderr
➜  cat stdout
This is stdout
➜  cat stderr
This is stderr

Setting exit codes

The exit code of lager python will be set to the exit code of your script. This can be used to check success or failure, which is particularly useful in a headless or automated environment such as Continuous Integration.

failure.py

print('Hello, world!')
raise SystemExit('Failed!')

Save this as failure.py. Then, we'll run it using lager python failure.py

➜  lager python failure.py
Hello, world!
Failed!
➜  echo $?
1

$? is a special shell environment variable that holds the exit status of the most recent command. Since it is non-zero, that means the program exited with an error.

Command line arguments

Command-line arguments can be sent to your lager python program. Use a -- to demarcate the end of the lager python arguments and the arguments for your program.

arguments.py

import sys
print(sys.argv[1:])

Save this as arguments.py. Then, we'll run it using lager python arguments.py -- arg1 arg2 arg3 --option 4 'foo bar'

➜  lager python arguments.py -- arg1 arg2 arg3 --option 4 'foo bar'
['arg1', 'arg2', 'arg3', '--option', '4', 'foo bar']

Environment variables

You can also set environment variables that will be accessible within your lager python program. There are two different ways to set environment variables - --env VAR=VALUE syntax is used to directly set a variable named VAR to VALUE. --passenv FOO can be used to pass the value of an environment variable named FOO that is already defined. This is especially useful with Continuous Integration, when you may have secrets that exist in your environment that you do not wish to expose on the command line

env.py

import os
print(f'FOO = {os.environ["FOO"]}')
print(f'BAZ = {os.environ["BAZ"]}')

Save this as env.py. Then, we'll run it using FOO=BAR lager python env.py --passenv FOO --env BAZ=qux.

➜  FOO=BAR lager python env.py --passenv FOO --env BAZ=qux
FOO = BAR
BAZ = qux