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why sys.stdout returned null #370
Description
Environment
- Pythonnet version: 2.2.2 (.NET 4, x64)
- Python version: 3.5
- Operating System: Windows 10
Details
Python is embedded in my .NET application using pythonnet. I found the output of the print statement in a python script is not showing on the screen until a "sys.stdout.flush()" called. I don't known how to redirect sys.stdout in .NET.
In order to call the flush() method directly in .NET, I tried
dynamic sys = PythonEngine.ImportModule("sys");
var stdout= sys.GetAttr("stdout"); // returns Noneand both sys.stdout and the variable stdout returned are None here.
In python, I tried
import sys
print(getattr(sys, 'stdout'))and it outputs a _io.TextIOWrapper object.
Why sys.stdout returned null here? How to make the output of python print statement show automatically?
Thanks.
Activity
den-run-ai commented on Feb 9, 2017
@yagweb can you try suggestions from here:
https://mail.python.org/pipermail/pythondotnet/2016-May/001777.html
http://stackoverflow.com/questions/37289082/redirect-stdout-stderr-from-python-embedded-in-net-application-to-text-box
https://mail.python.org/pipermail/pythondotnet/2016-May/001776.html
We should probably include some API to make this easier, since some people are already using this in ad-hoc ways.
den-run-ai commented on Feb 9, 2017
in your example make sure you acquire the GIL with using (Py.GIL()) {} and try this:
yagweb commented on Feb 9, 2017
@denfromufa Yes my code was put in the curly braces of '''using (Py.GIL()) {}'''. I have tried your code snippet, but the stdout is still null. I'm not sure whether it's a problem of CPython or pythonnet. Maybe somebody else can try it with the other versions of CPython to get more clues.
For redirecting stdout, 3 solutions are found according to these links, but the most elegant one (similar to the 3rd solution) provided by Cameron Hayne is not working because of the limitation of pythonnet. It needs “make any .NET object can be passed to CPython”. Here I give a briefly description, hope they can help with the API design.
Solution 1: Not automatically, so not suggested.
using (Py.GIL()) { PythonEngine.RunSimpleString("import sys\n"+ "from io import StringIO\n"+ "sys.stdout = mystdout = StringIO()\n"); //test PythonEngine.RunSimpleString("print('Hello!')"); Console.Write(stdout.getvalue()); }Solution 2: Make Python call Console.Write automatically.
using (Py.GIL()) { PythonEngine.RunSimpleString(@" import sys from System import Console class output(object): def write(self, msg): Console.Write(msg) def writelines(self, msgs): for msg in msgs: Console.Write(msg) def flush(self): pass def close(self): pass sys.stdout = sys.stderr = output() "); //test PythonEngine.RunSimpleString("print('Hello!')"); }Solution 3
This solution is similar to the one proposed by Cameron Hayne. Define a .NET class that implements the method expected of a Python file object, then create an instance and set it to sys.stdout.
using (Py.GIL()) { PythonEngine.RunSimpleString(@" import sys from Test import Output sys.stdout = sys.stderr = Output() "); //test PythonEngine.RunSimpleString("print('Hello!')"); }public class Output { public void write(String str) { Console.Write(str); } public void writelines(String[] str) { foreach (String line in str) { Console.Write(str); } } public void flush(){} public void close(){} }Solution 4: The best one but not working now.
A more elegent version of solution 3 proposed by Cameron Hayne looks like
Output output = new Output(); using (Py.GIL()) { dynamic sys = Py.Import("sys"); sys.stdout = output; sys.stderr = output; }Howerver, Exception is throw when assigning sys.stdout because variable output is not of type PyObject. Make the Output class inherit from PyObject does not work. A possible solution is updating the TrySetMember method of PyObject as below. Make any .NET object can be passed to CPython is very attractive. Solution 3 may provide some help.
public override bool TrySetMember(SetMemberBinder binder, object value) { if (this.HasAttr(binder.Name)) { if (value is PyObject) { //some wrapping work. Can also be used for passing any .NET object as argument of PyFunction //Limited by my knowledge about pythonnet. } else { this.SetAttr(binder.Name, (PyObject)value); return true; } } else { return base.TrySetMember(binder, value); } }It’s worth to mention that, after set the sys.stdout in the three solutions, the stdout can be obtained normally
dynamic sys = Py.Import("sys"); var stdout = sys.stdout; //now it‘s not null!filmor commented on Feb 9, 2017
I like that last proposal a lot. Shouldn't be too difficult.
den-run-ai commented on Feb 9, 2017
@filmor @yagweb so you like this proposal to pass any .NET object to CPython without conversion? If yes, here is related issue:
#94
The 1st solution is what Python BDFL Guido recommends when embedding CPython:
https://docs.python.org/3/faq/extending.html#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr
yagweb commented on Feb 10, 2017
@denfromufa @filmor I think this proposal is different from #94
This one talks about “pass any .NET object to CPython” and that one talks about “pass any CPython object to .NET”.
Both of them should take care about the reference management and efficiency.
I read the source code of converter.cs and found a way to do so. The PR has be committed. Several tests have conducted for this modification.
den-run-ai commented on Feb 12, 2017
@yagweb @filmor I like this 4th proposal as well. let's continue the relevant discussion in the pull request.
we can keep this issue open to find out why var stdout= sys.GetAttr("stdout"); // returns None.
filmor commented on Feb 12, 2017
We don't need to keep this open, this is documented behaviour:
https://docs.python.org/3/library/sys.html#sys.__stdin__
yagweb commented on Feb 14, 2017
@filmor @denfromufa I agree with this explanation. I have a proposal based on this mechanism.
When we embedded Python in a .NET Console application. Usually we want the Python print statement outputs to the screen directly.
So, what about the function PythonEngine.Initialize automatically redirects the sys.stdout and sys.stderr to the Console when it detecting the current application is a Console application just like the Python does, and the Solution 4 can be used here.
Or, add an argument "bool isredirect=false" to PythonEngine.Initialize, when the Initialize function is called with isredirect=true, it redirects the sys.stdout and sys.stderr to the Console using Solution 4.
yagweb commented on Feb 14, 2017
@filmor @denfromufa If the Solution 4 is chosen as the recommended way for the sys.stdout redirection. It's better to provide a .NET interface listing the methods needed. It can help us when we want the redirection in other senses.
den-run-ai commented on Feb 14, 2017
@yagweb i agree with this suggestion, and you @filmor?
Similar setup is provided for sys.argv in Initialize.
filmor commented on Feb 14, 2017
I agree with that, too. However, there doesn't seem to be a simple way to detect whether an application is running from a console in .NET. Also, the override should probably be done by wrapping Console.Out etc. instead of using the Console.Write functions.
yagweb commented on Feb 11, 2018
For any one who maybe interested in this topic, I add my PySysIO class here.
No automatically redirect, it can be used like this, or in this way.
// redirect to Console.Out, I usually put this at the entrance of my program. PySysIO.ToConsoleOut(); //test using (Py.GIL()) { PythonEngine.RunSimpleString("print('hello ConsoleOut!')"); }