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{ | ||
"cells": [ | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"# Capturing C-level stdout/stderr with `wurlitzer`\n", | ||
"\n", | ||
"Sometimes in Python you are calling some C code.\n", | ||
"Sometimes that C code makes calls to `printf`,\n", | ||
"or otherwise writes to the stdout/stderr of the process." | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 1, | ||
"metadata": { | ||
"collapsed": false | ||
}, | ||
"outputs": [], | ||
"source": [ | ||
"import ctypes\n", | ||
"libc = ctypes.CDLL(None)\n", | ||
"\n", | ||
"try:\n", | ||
" c_stderr_p = ctypes.c_void_p.in_dll(libc, 'stderr')\n", | ||
"except ValueError:\n", | ||
" # libc.stdout is has a funny name on OS X\n", | ||
" c_stderr_p = ctypes.c_void_p.in_dll(libc, '__stderrp')\n", | ||
"\n", | ||
"\n", | ||
"def printf(msg):\n", | ||
" \"\"\"Call C printf\"\"\"\n", | ||
" libc.printf((msg + '\\n').encode('utf8'))\n", | ||
"\n", | ||
"def printf_err(msg):\n", | ||
" \"\"\"Cal C fprintf on stderr\"\"\"\n", | ||
" libc.fprintf(c_stderr_p, (msg + '\\n').encode('utf8'))" | ||
] | ||
}, | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"IPython forwards the Python-level `sys.stdout` and `sys.stderr`,\n", | ||
"but it leaves the process-level file descriptors that C code will write to untouched.\n", | ||
"That means that in a context like this notebook, these functions will print to the terminal, because they are not captured:" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 2, | ||
"metadata": { | ||
"collapsed": true | ||
}, | ||
"outputs": [], | ||
"source": [ | ||
"printf(\"Hello?\")\n", | ||
"printf_err(\"Stderr? Anybody?\")" | ||
] | ||
}, | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"With wurlitzer, we can capture these C-level functions:" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 3, | ||
"metadata": { | ||
"collapsed": true | ||
}, | ||
"outputs": [], | ||
"source": [ | ||
"from wurlitzer import pipes, sys_pipes, STDOUT, PIPE" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 4, | ||
"metadata": { | ||
"collapsed": true | ||
}, | ||
"outputs": [], | ||
"source": [ | ||
"with pipes() as (stdout, stderr):\n", | ||
" printf(\"Hello, stdout!\")\n", | ||
" printf_err(\"Hello, stderr!\")" | ||
] | ||
}, | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"and redisplay them if we like:" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 5, | ||
"metadata": { | ||
"collapsed": false | ||
}, | ||
"outputs": [ | ||
{ | ||
"name": "stdout", | ||
"output_type": "stream", | ||
"text": [ | ||
"Hello, stdout!\n" | ||
] | ||
}, | ||
{ | ||
"name": "stderr", | ||
"output_type": "stream", | ||
"text": [ | ||
"Hello, stderr!\n" | ||
] | ||
} | ||
], | ||
"source": [ | ||
"import sys\n", | ||
"sys.stdout.write(stdout.read())\n", | ||
"sys.stderr.write(stderr.read())" | ||
] | ||
}, | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"Some tools, such as the IPython kernel for Jupyter,\n", | ||
"capture the Python-level `sys.stdout` and `sys.stderr` and forward them somewhere.\n", | ||
"In the case of Jupyter, this is over a network socket, so that it ends up in the browser.\n", | ||
"\n", | ||
"If we know that's going on, we can easily hook up the C outputs to the Python-forwarded ones with a single call:" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 6, | ||
"metadata": { | ||
"collapsed": false | ||
}, | ||
"outputs": [ | ||
{ | ||
"name": "stdout", | ||
"output_type": "stream", | ||
"text": [ | ||
"Hello from C, 0!\n", | ||
"Hello from C, 1!\n", | ||
"Hello from C, 2!\n", | ||
"Hello from C, 3!\n", | ||
"Hello from C, 4!\n" | ||
] | ||
} | ||
], | ||
"source": [ | ||
"import time\n", | ||
"\n", | ||
"with sys_pipes():\n", | ||
" for i in range(5):\n", | ||
" time.sleep(1)\n", | ||
" printf(\"Hello from C, %i!\" % i)\n" | ||
] | ||
}, | ||
{ | ||
"cell_type": "markdown", | ||
"metadata": {}, | ||
"source": [ | ||
"We can also capture the pipes to any writeable streams, such as a `StringIO` object:" | ||
] | ||
}, | ||
{ | ||
"cell_type": "code", | ||
"execution_count": 7, | ||
"metadata": { | ||
"collapsed": false | ||
}, | ||
"outputs": [ | ||
{ | ||
"name": "stdout", | ||
"output_type": "stream", | ||
"text": [ | ||
"Hello, stdout!\n", | ||
"Hello, stderr!\n", | ||
"\n" | ||
] | ||
} | ||
], | ||
"source": [ | ||
"import io\n", | ||
"\n", | ||
"stdout = io.StringIO()\n", | ||
"with pipes(stdout=stdout, stderr=STDOUT):\n", | ||
" printf(\"Hello, stdout!\")\n", | ||
" printf_err(\"Hello, stderr!\")\n", | ||
"\n", | ||
"print(stdout.getvalue())" | ||
] | ||
} | ||
], | ||
"metadata": { | ||
"kernelspec": { | ||
"display_name": "Python 3", | ||
"language": "python", | ||
"name": "python3" | ||
}, | ||
"language_info": { | ||
"codemirror_mode": { | ||
"name": "ipython", | ||
"version": 3 | ||
}, | ||
"file_extension": ".py", | ||
"mimetype": "text/x-python", | ||
"name": "python", | ||
"nbconvert_exporter": "python", | ||
"pygments_lexer": "ipython3", | ||
"version": "3.5.1" | ||
} | ||
}, | ||
"nbformat": 4, | ||
"nbformat_minor": 1 | ||
} |