🌐 Mailman 3 July 2019 - Python-list - python.org

Nesting Custom Errors in Classes
by DL Neil July 23, 2019

July 23, 2019

Do you use nested classes? [following-on from the earlier, "Namespaces: memory vs 'pollution'" discussion thread, wherein a certain 'someone' remembered to from ... import ... as ... an 'action' class but forgot to also import the related custom error class! The original quest was for a wild-card import device. This discussion may obviate continuing the quest and/or needing to remember...] Python manages nested classes. I've NEVER seen such 'in the wild'. (but perhaps I lead a sheltered life?) What are proposed as use-cases for such a technique? - other languages may not offer decent "inheritance", and this is an alternative method/hack - the class is a 'class factory', generating/returning an object ? any others Why not use it as an "encapsulation" device? (please be gentle - reminder: I am too old to have been an OO-native!) ***** stub definitions >>> class PythonEnvironment(): ... class PythonVersionError( EnvironmentError ): ... pass ... def iscompatible( self ): ''' Ensure that the Python in-use will support all of the facilities employed by the application. ''' ... # stub to simulate failure ... raise self.PythonVersionError ... (code would require an import or from ... import ...) >>> pe = PythonEnvironment() ***** its basic application becomes:- >>> pe.iscompatible() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 6, in compatibility __main__.PythonVersionError ***** somewhat more realistic use:- >>> try: ... pe.iscompatible() ... except PythonEnvironment.PythonVersionError: ... print( "Trapped! -> informative errmsg" ) ... Trapped! -> informative errmsg With this construct, one only has to import the 'outer' class, rather than both the class AND its ancillary error class! Why haven't I seen it before? Can you see anything 'wrong' with this picture? -- Regards, =dn

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Re: Embedding Python in C
by Jesse Ibarra July 23, 2019

July 23, 2019

On Saturday, July 20, 2019 at 1:11:51 PM UTC-6, Stefan Behnel wrote: > Jesse Ibarra schrieb am 20.07.19 um 04:12: > > Sorry, I am not understanding. Smalltlak VW 8.3 does not support Python. > > I can only call Pyhton code through C/Python API. > > Ok, but that doesn't mean you need to write code that uses the C-API of > Python. All you need to do is: > > 1) Start up a CPython runtime from Smalltalk (see the embedding example I > posted) and make it import an extension module that you write (e.g. using > the "inittab" mechanism [1]). > > 2) Use Cython to implement this extension module to provide an interface > between your Smalltalk code and your Python code. Use the Smalltalk C-API > from your Cython code to call into Smalltalk and exchange data with it. > > Now you can execute Python code inside of Python and make it call back and > forth into your Smalltalk code, through the interface module. And there is > no need to use the Python C-API for anything beyond step 1), which is about > 5 lines of Python C-API code if you write it yourself. Everything else can > be implemented in Cython and Python. > > Stefan > > > [1] > https://docs.python.org/3/extending/embedding.html?highlight=PyImport_appen… This cleared so much @Stefan, thank you. I just need some clarification if you don't mind. In (1), when you say "import an extension module that you write", do you mean the Python library that was created "import emb"? Is that gonna be written in Cython or standalone .C file? in (2), what do to mean when you said "Use the Smalltalk C-API from your Cython code to call into Smalltalk and exchange data with it."? Please advise, Thank you so much for your help.

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Re: Nesting Custom Errors in Classes
by Rob Gaddi July 23, 2019

July 23, 2019

On 7/22/19 7:54 PM, DL Neil wrote: > Do you use nested classes? > > [following-on from the earlier, "Namespaces: memory vs 'pollution'" discussion > thread, wherein a certain 'someone' remembered to from ... import ... as ... an > 'action' class but forgot to also import the related custom error class! The > original quest was for a wild-card import device. This discussion may obviate > continuing the quest and/or needing to remember...] > > > Python manages nested classes. > > I've NEVER seen such 'in the wild'. > (but perhaps I lead a sheltered life?) > > > What are proposed as use-cases for such a technique? > - other languages may not offer decent "inheritance", and this is an alternative > method/hack > - the class is a 'class factory', generating/returning an object > ? any others > > > Why not use it as an "encapsulation" device? > (please be gentle - reminder: I am too old to have been an OO-native!) > > > ***** stub definitions > > >>> class PythonEnvironment(): > ... class PythonVersionError( EnvironmentError ): > ... pass > ... def iscompatible( self ): > ''' Ensure that the Python in-use will support > all of the facilities employed by the application. > ''' > ... # stub to simulate failure > ... raise self.PythonVersionError > ... > > (code would require an import or from ... import ...) > >>> pe = PythonEnvironment() > > > ***** its basic application becomes:- > > >>> pe.iscompatible() > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > File "<stdin>", line 6, in compatibility > __main__.PythonVersionError > > > ***** somewhat more realistic use:- > > >>> try: > ... pe.iscompatible() > ... except PythonEnvironment.PythonVersionError: > ... print( "Trapped! -> informative errmsg" ) > ... > Trapped! -> informative errmsg > > > With this construct, one only has to import the 'outer' class, rather than both > the class AND its ancillary error class! > > > Why haven't I seen it before? Can you see anything 'wrong' with this picture? I've used them sometimes for basic encapsulation principles without really gaining anything. I use inheritance of nested classes in my https://pypi.org/project/indexedproperty/ project, where subclasses of IndexedProperty have a _Trampoline inner class subclassed from Trampoline that can be redefined as needed, but that's a bit of an obscure use case. -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix.

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Re: Creating time stamps
by Michael F. Stemper July 23, 2019

July 23, 2019

On 22/07/2019 15.58, Chris Angelico wrote: > On Tue, Jul 23, 2019 at 6:34 AM Michael F. Stemper > <michael.stemper(a)gmail.com> wrote: >> >> I have some code that generates a time-stamp as follows: >> >> from datetime import datetime >> tt = datetime.now() >> timestamp = "%4d-%02d-%02d %02d:%02d" % \ >> (tt.year, tt.month, tt.day, tt.hour, tt.minute) >> >> I later realized that I could have written it as: >> >> from datetime import datetime >> from time import strftime >> timestamp = datetime.now().strftime( "%Y-%m-%d %H:%M" ) >> >> The first seems a little clunky with its accessing of multiple >> attributes, but the second has an additional import. Is there >> any reason to prefer one over the other? > > What's the second import doing though? You never use strftime. Cleaned my contacts, cleaned my readers. Still see strftime() in the third line. Tried to run the code without the second line. It ran without complaint. Apparently, the strftime() in that last line is not the one that I explicitly imported, but a method of datetime.now(). Did I get that right? > I'd go > with the second one, but with just a single import. Sounds like a winner to me. Thanks. -- Michael F. Stemper 87.3% of all statistics are made up by the person giving them.

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Namespaces: memory vs 'pollution'
by DL Neil July 23, 2019

July 23, 2019

How do you remember to from-import- 'everything' that is needed? I have a 'utility module' which contains a bunch of classes which examine/check/log aspects of the execution environment. One of which is PythonEnvironment, another relates to the HostSystem (as examples). They are most-frequently used to ensure that an application runs within an appropriate environment, eg don't use Posix code on MS-Windows and don't call for v3.7 features from Python3.5. Today, I 'grabbed' the module, imported the PythonEnvironment, and PyTested the application code's Python version - as much for my amusement as anything else I asked for an impossible versionNR. Of course, the test failed. Upon closer inspection, I realised it didn't just fail; it failed badly! Some silly, little, boy had imported the PythonEnvironment class but failed to ALSO import PythonVersionError. So, the reported error was not the expected exception! Yes, the two classes appear in the module one-above-the-other, and yes, in the module's own pytest import-ing both classes is 'right there' in front of my nose. So, not my finest hour! Is there some 'easy way' to make sure that one doesn't just import the desired class, but also remembers to import 'everything else' that might be necessary. In this case, it'd be rather nice to: from environment_module import Python* NB this is a syntax error, and won't import both the PythonEnvironment and PythonVersionError classes. NBB I could import the module, fish-around in its __dict__, and do something like update-ing locals() with every key containing "Python" - but that is surely a truly, ugly, hack. The 'lazy' answer is (purists, look away now!): from environment_module import * but that import-s "everything, including the kitchen sink"! After you've recovered from the shock-horror of thinking about such blatant "namespace pollution", if the module only contains a bunch of classes, is it really such an evil act? (and if the module's import-as-abbreviations are moved 'inside' the relevant class, those 'disappear' from the global namespace reducing the risk of name-clashes down to the names of the custom-classes and the PSL imports, eg os and platform) What do you do to (respecting purism) ensure 'everything' (necessary) is imported (and nothing more), preferably without relying upon (faulty, in my case) human-memory or reading through volumes of code/documentation? -- Regards, =dn

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How to execute shell command in Python program?
by Madhavan Bomidi July 23, 2019

July 23, 2019

Hi, I have the following shell commands to run my executable created with gfortran: ----------------------------------------------------------------------------- $ pwd /OPAC/opac31a/opac31 $ ls extback.dat opac opac.f result terr.dat input opac.cfg pyopac3.py solar.dat wel.dat $ ./opac <====== ******************************************************** * Optical Properties of Aerosols and Clouds OPAC 3.1 * * -------------------------------------------------- * * * * The contents of OPAC has been described in: * * M. Hess, P. Koepke and I. Schult (1997): * * "Optical Properties of Aerosols and Clouds: * * The software package OPAC" * * submitted to Bull. Am. Meteor. Soc. * * * * last update: 14.11.97 M. Hess * ******************************************************** name of input file (without extension .inp), (max. 8 characters) default: opac.inp opac <====== input file input/opac.inp used READY. Note: The following floating-point exceptions are signalling: IEEE_DENORMAL ----------------------------------------------------------------------------------- Now my concern is the following commands (pointed by <======) to be run within the Python program. I tried to use the following lines in the Python program: --------------------- import subprocess subprocess.call(['./opac'],shell=True) Out[21]: 2 I am not able to know how I input the 'opac' (which is the input filename) Please help me.

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PyPi twine check does not like my README.rst files anymore
by Barry Scott July 23, 2019

July 23, 2019

I am update some PyPI projects and found that twine was refusing the upload. I have been googling and reading the PyPI guides but did not find anthing that helps me understand what is required. What am I doing wrong? Using the twine check command I get reports about the reasons. All the code on https://github.com/barry-scott/namedstruct where you can see the README.rst rendered in full. Where as at https://pypi.org/project/namedstruct/ only the first line is rendered. Checking distribution dist\namedstruct-1.2.1-py3-none-any.whl: warning: `long_description_content_type` missing. defaulting to `text/x-rst`. Passed Checking distribution dist\namedstruct-1.2.1.tar.gz: warning: `long_description_content_type` missing. defaulting to `text/x-rst`. Failed The project's long_description has invalid markup which will not be rendered on PyPI. The following syntax errors were detected: line 1: Severe: Unexpected section title or transition. The part of setup.py that does this is: setup( name='namedstruct', version=open('version.txt').read().strip(), description='namedstruct encapsulates struct.unpack() with results accessed by name', long_description=long_description, long_description_content_type='text/x-rst', # The project's main homepage. url='https://github.com/barry-scott/namedstruct', Where should I have added the content type? The other reason is the show stopper. It does not allow me to have sections in my README.rst. The README.rst starts with: Module namedstruct ------------------ namedstruct encapsulates struct.unpack() and struct.pack() with results accessed by name. Classes ------- class namedstruct.namedstruct If I remove the sections the README.rst shown only has the first line of the readme twine allows be to upload but only the first line of the README is rendered. Barry

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Hermetic environments
by DL Neil July 23, 2019

July 23, 2019

Is Python going 'the right way' with virtual environments? Warning: this post may contain Python-heresy. I don't use any of the Python virtual environments! I'm not an *ops person either - unless Ops happened to be included within a wider department. However, in attempting to follow the "Proper shebang for python3" thread, the following appealed to my mind: <<< The truth is that there is no single answer to this problem until you hermetically seal an environment. Docker comes close, a freestanding VM does this most completely. But short of that, the ability to decide to use something other than system-provided tools absolutely IS a requirement in systems of any scale. >>> I do attempt to "hermetically seal" work, building a separate Virtual Box VM for each customer (and usually more than one). To be clear, this is less about Python (I use other languages), and more a method of keeping things completely separate, for privacy and security. It also allows me to conveniently separate 'dev' from 'prod'/support. I don't 'fiddle' but leave all the executables where the OpSys loads them (smarter minds than mine!), and load/pip everything as system-wide (no thinking necessary!). With the power of today's machines, I don't really notice* any performance degradation. There is no hassle about where the binaries may be found. There is no issue if one client is sticking with an earlier version but another wants the next iteration of development using some v3.7+ advancement. Similarly, there's no chance of tripping-over other clients' feet because some are web-based, some GUI, and some cmdLN! * we recently ran a test on a fairly compute-heavy statistics application: the same system on my (relatively old) portable and VBox, against a 'delivered version' installed natively on a newer and much bigger (RAM and CPU) Windows 10 box. Guess which little, old, Fedora-Linux, VBox-slowed machine 'won'!? Whilst the VM imposes an overhead 'cost', because it can be stripped-down/lean-and-mean/nothing-that's-unnecessary, the VM seems to be cost-effective. (regardless of the above tactics and criteria!) Standing on principle, I'd like to use a system other than VBox (Oracle) but it is neatly cross-platform and multi-platform. When eval-time is available, Docker is definitely on the try-out list (as mentioned in the quote, above). Normally, 'delivery' is a matter of duplicating the VM and fitting it into the client's DNS. It is similarly easy to pass work to A.N.Other, and presents no problem if there are multiple developers. (providing THE VM is not (only) on my portable...) Am I 'getting away with it', perhaps because my work-pattern doesn't touch some 'gotcha' or show-stopper? Why, if so much of 'the rest of the world' is utilising "containers", both for mobility and for growth, is the Python eco-system following its own path? Is there something about dev (and ops) using Python venvs which is a significant advantage over a language-independent (even better: an OpSys-independent) container? -- Regards, =dn

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Transfer Image from Raspberry Pi (Python) to Android app (Java)
by rkartunova＠yahoo.com July 22, 2019

July 22, 2019

Hi everyone! I need help transferring an image via TCP from a python program on my raspberry pi to an android application. I have set up a client-server architecture such that my raspberry pi 3 records audio, performs some analysis on it, and then sends the data (via TCP) to the android app to display on the app screen. The recording and analysis is done and I am able to make the connection and transfer string data that displays on the app with no problem. However, I have been unsuccessful in transferring an image from rpi to android app. So basically, the image is stored on the rpi and I an attempting to transfer the image to the app to display it. I have been working on this for over a week with no luck so any help would be greatly appreciated! My current implementation (code snippets provided below): On rpi (python): Like I said, sending strings and displaying them on the android app is done without any problem. When I am sending the image portion of the audio analysis, I send a string first that says "?start" so that the android side knows that an image instead of a string is about to be sent (and will wait to update the GUI until it receives the entire image). Then, I open the image stored on rpi and read the entire image as a byte array (typically about 40-50k bytes). I get the length of the byte array and send that as a string to android app. Finally, I send the byte array to the android and it waits for an OK message from the app. All of this works without reporting any errors. On android app (java): When the app receives the "?start" string, it then uses a Buffered Reader (which is what I used to read the string data I had transferred to the app successfully earlier) to read the size of the image byte array. Then, I create 2 buffers, msg_buff and img_buff. msg_buff will read in 1024 bytes at a time while img_buff will hold the entire byte array of the image. In the infinite while loop, I have a DataInputStream, called in, read bytes into msg_buff and returns the number of bytes read. Then, I concatenate the copied contents of msg_buff into img_buff. Once the bytes read from in is -1 or the img_offset (which is just the total number of bytes read) is greater than or equal to the size of the image bytes array, the while loop is broken. Then, I would attempt to save the image to android internal storage and then load it later to an imageView to display it. This code does successfully read in the bytes until there are around 2000-3000 bytes left to be read and then it seems to freeze on the int bytes_read = in.read(msg_buff, 0, msg_buff.length) line. I have not been able to get past that point so I do not know if saving the image to internal storage and then loading it to imageview that way will work either. I have also tried using base64 encoding/decoding but that also kept producing errors. I have tried rpi only sending 1024 bytes of the image at a time but that also did not work. I have tried several implementations of this approach but nothing has worked so far. If anyone sees anything wrong or has another approach, I am all ears! Android Studio (app side): //receives the message which the server sends back InputStream sin = socket.getInputStream(); OutputStream sout = socket.getOutputStream(); DataInputStream in = new DataInputStream(sin); mBufferIn = new BufferedReader(new InputStreamReader(socket.getInputStream())); //in this while the client listens for the messages sent by the server while (mRun) { mServerMessage = mBufferIn.readLine(); if (mServerMessage != null && mMessageListener != null) { //Check if data is image if(mServerMessage.equals("?start")) { // Get length of image byte array int size = Integer.parseInt(mBufferIn.readLine()); // Create buffers byte[] msg_buff = new byte[1024]; byte[] img_buff = new byte[size]; int img_offset = 0; while(true){ int bytes_read = in.read(msg_buff, 0, msg_buff.length); if(bytes_read == -1){ break; } //copy bytes into img_buff System.arraycopy(msg_buff, 0, img_buff, img_offset, bytes_read); img_offset += bytes_read; if( img_offset >= size) { break; } } ContextWrapper cw = new ContextWrapper(ApplicationContextProvider.getContext()); File directory = cw.getDir("imageDir", Context.MODE_PRIVATE); File mypath = new File(directory, "signal.jpeg"); Bitmap bitmap = BitmapFactory.decodeByteArray(img_buff, 0, img_buff.length); FileOutputStream fos = null; try{ fos = new FileOutputStream(mypath); //Use compress method on Bitmap object to write image to OutputStream bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos); //Send OK byte[] OK = new byte[] {0x4F, 0x4B}; sout.write(OK); } catch (Exception e) { e.printStackTrace(); } } } Raspberry Pi (python): def image_to_byte_array(image_file, conn): with open(image_file, 'rb') as imageFile: content = imageFile.read() conn.sendall("?start\n".encode('utf-8')) size = len(content) strSize = str(size) + "\n" conn.sendall(strSize.encode('utf-8')) conn.sendall(content) Note that conn is the connection between the app and the rpi and the images are PNG. If anyone knows why this isn't working or has a better way for me to do this, I would greatly appreciate it!! Thank you in advance!!

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Proper shebang for python3
by Manfred Lotz July 22, 2019

July 22, 2019

Hi there, Pretty new to python I've got a question regarding the proper shebang for Python 3. I use #!/usr/bin/python3 which works fine. Today I saw #!/usr/bin/python3 -tt and was wondering what -tt means. Being on Fedora 30, Python 3.7.3 the man page of python3 doesn't even mention -t. python 2 man page mentions -t Issue a warning when a source file mixes tabs and spaces for indentation in a way that makes it depend on the worth of a tab expressed in spaces. Issue an error when the option is given twice. I guess that -t has the same meaning with python 3.7.3. My questions: 1. Is my guess correct? 2. Is it a bug that it is not mentioned? python3 --help doesn't mention it either. -- Manfred

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