QByteArray to string?

SGaist

@jazzycamel long time no see ! Thanks for the thorough explanation :-)
Parts of it would be a welcome addition to the PyQt5 documentation.

jazzycamel

@JNBarchan
There is indeed no such thing as QString() in PyQt5. It shouldn't be necessary as the library takes care of type marshalling between the Python and Qt (C++) types. In fact, while there is a QVariant(), its generally not necessary to use it for the same reason. QByteArray() does exist also, but I would steer clear of it if possible and let PyQt5 deal with via bytes().

No, I will never (and no one else should!) ever make Python like C#!! :)

JonB

@jazzycamel , or anyone else

Having implemented qba.data().decode('utf8') as directed, I have now come across a situation where the QByteArray data returned by QProcess.readAllStandardOutput() from an OS command run under Windows causes the Python/PyQt code to generate a UnicodeDecodeError error, as detailed in my post https://forum.qt.io/topic/85493/unicodedecodeerror-with-output-from-windows-os-command

This makes it impossible to convert the data, blocking the whole behaviour of my usage.

My belief is that this would not be happening at all from C++ where I would simply use whatever methods of QByteArray/QString or the language. The problem is precisely is that I am being forced to use a "Python/PyQt" way of doing this, causing the error in Python/PyQt only, which is exactly why I didn't want to have to do that but cannot get access to the necessary types/methods of Qt from PyQt...?

SGaist

Can you show the code you use ?

JonB

@SGaist
I promise you all you'll see is a QByteArray being returned with the sub-process's output, and I'm trying to convert that to a QString to put into a QTextEdit. That's all the question is. And I get a UnicodeDecodeError, probably when robocopy echoes the name of a file which has that 0x9c character in it via PyQt's decode():

can't decode byte 0x9c in position 32: invalid start byte

So presumably all you have to do is create a QByteArray, put a 0x9c in its first byte, and try qba.data().decode('utf8'). That's what this thread is about.

This whole issue where I'm discussing the code is in https://forum.qt.io/topic/85493/unicodedecodeerror-with-output-from-windows-os-command. If you'd be kind enough to look at that, I think that's a more appropriate place to discuss the code than here? If you still want more code there, let me know, and I'll supply.

SGaist

I don't have a Windows machine at hand. Doing this on macOS yields correct results

from PyQt5.QtCore import QByteArray
ba = QByteArray()
ba.append(u"\u009C")
PyQt5.QtCore.QByteArray(b'\xc2\x9c')
ba.data().decode('utf-8')
'\x9c'
ba.data().decode('utf-16')
'鳂'

JonB

@SGaist
I'm afraid I don't believe that relates to the situation.

I now have information from the client:

The exception occurs (only) when a filename robocopy encounters --- robocopy is echoing filenames as it goes --- contains the £ (UK pound sterling) character (I am in the UK, you may not be). In that situation, ba.data().decode('utf-8') (where ba is the QByteArray from QProcess.readAllStandardOutput()) results in:

Unhandled Exception:

'utf-8' codec can't decode byte 0x9c in position 32: invalid start byte

<class 'UnicodeDecodeError'>
File "C:\HJinn\widgets\messageboxes.py", line 289, in processReadyReadStandardOutput
output = output.data().decode('utf-8')

Now, armed with that information:

In a Command Prompt I type in: echo £ > file
I dump the file and I see: 9C 20 0D 0A
So the £ character is single byte with value 0x9C

SGaist

What do you get if you use unicode_escape in place of utf-8 ?

JonB

@SGaist
I don't know, because I don't have access to the code right now, but I will tomorrow.

Thank you, your suggestion is much more like what I have been looking for. We are now discussing the argument to decode():

I believe utf-8 is definitely right for Linux, where I develop.
I'm beginning to learn (whether I like it or not) that it is not for Windows.
Under Windows utf-8 does work 99% of the time, but not always, and now I know not for the £ character.
I believe that either latin-1 or windows_1252 may be able to handle this correctly.
I will also try your unicode_escape if you think it's worthwhile.

JonB

@SGaist
I believe what I am seeking from you is: Haven't I seen that Qt has some function to "get the current system encoding", but I can't spot it?

Then my code would be:

ba.data().decode(Qt.getCurrentSystemEncoding())

and everything would just work....

[EDIT: Ooohhhh, is http://doc.qt.io/qt-5/qtextcodec.html#codecForLocale what I'm looking for, perhaps?

QTextCodec *QTextCodec::codecForLocale()

Returns a pointer to the codec most suitable for this locale.

On Windows, the codec will be based on a system locale. On Unix systems, the codec will might fall back to using the iconv library if no builtin codec for the locale can be found.

Or, was I thinking of the Python sys.getfilesystemencoding() https://docs.python.org/3/library/sys.html#sys.getfilesystemencoding
But that seems filename-specific, my output could be anything, not especially file names.

JonB

[This post cross-posed to https://forum.qt.io/topic/85493/unicodedecodeerror-with-output-from-windows-os-command/18 ]

For the record, I have done exhaustive investigation, and there is only one solution which "correctly" displays the £ character under Windows. I am exhausted so will keep this brief:

To create a file name with a £ in it: Go into, say, Notepad and use its Save to name a file like abc£.txt. This is in the UK, using a UK keyboard and a standard UK-configured Windows.
Note that at this point if you view the filename in either Explorer or, say, via dir you do see a £, not some other character. That's what my user will want to see in the output of the command he will run.
Run an OS command like robocopy or even dir, which will include the filename in its output.
Read the output with QProcess.readAllStandardOutput(). I'm saying the £ character will arrive as a single byte of value 0x9c.
For the required Python/PyQt decoding bytes->str (QByteArray->QString) line, the only thing which works (does not raise an exception) AND represents the character as a £ is: ba.bytes().decode("cp850").

That is the "Code Page 850", used in UK/Western Europe (so I'm told). It is the result output of you open a Command Prompt and execute just chcp.

Any other decoding either raises UnicodeDecodeError (e.g. if utf-8) or decodes but represents it with another character (e.g. if windows_1252 or cp1252).

I still haven't found a way of getting that cp850 encoding name programatically from anywhere --- if you ask Python for, say, the "system encoding" or "user's preferred encoding" you get the cp1252 --- so I've had to hard-code it. [EDIT: If you want it, it's ctypes.cdll.kernel32.GetConsoleOutputCP().]

So there you are. I don't have C++ as opposed to Python for Qt, but I have a suspicion that if anyone tries it using the straight C++ Qt way of text = QString(process.readAllStandardOutput()) they'll find they do not actually get to see the £ symbol....

germyrinn

Python makes a clear distinction between bytes and strings . Bytes objects contain raw data — a sequence of octets — whereas strings are Unicode sequences . Conversion between these two types is explicit: you encode a string to get bytes, specifying an encoding (which defaults to UTF-8); and you decode bytes to get a string. Clients of these functions should be aware that such conversions may fail, and should consider how failures are handled.

We can convert bytes to string using bytes class decode() instance method, So you need to decode the bytes object to produce a string. In Python 3 , the default encoding is "utf-8" , so you can use directly:

b"python byte to string".decode("utf-8")

JonB

@germyrinn
Hi, this was an old post of mine.

As I wrote, the problem is that for the £ sign e.g. read from a file created in the way I describe, decode("utf-8") gives me a UnicodeDecodeError. I found the only conversion which works is decode("cp850").