Python

Problem:

When trying to run a Python script using PyVISA with a USB instrument, you see the following error message:

ValueError: Please install PyUSB to use this resource type.
No module named 'usb'

Solution:

On Linux distributions such as Ubuntu, you can fix this using

sudo apt-get -y install python3-usb

This will install the Python3 usb package and the libusb system packages.

In our previous post Matplotlib custom SI-prefix unit tick formatter we showed how to format a matplotlib Y axis with a custom unit with SI prefixes.

Similarly, we can use UliEngineering.Math.Decibel in order to format plots (most notably plots with a logarithmic Y axis) as decibels:

from UliEngineering.Math.Decibel import *
import matplotlib.ticker as mtick

def decibel_formatter(v0=1.0, unit='dB'):
    def format_value(value, pos=None):
        dB = value_to_dB(value, v0=v0)
        return f'{dB:.0f} {unit}'
    return format_value

# Usage example:
plt.gca().set_yscale("log") # Optional

plt.gca().yaxis.set_major_formatter(mtick.FuncFormatter(decibel_formatter()))

The following code will iterate over all IP addresses in the given network, i.e. 192.168.1.0 ... 192.168.1.254:

import ipaddress

network = ipaddress.ip_network('192.168.1.0/24')

for ip in network:
    print(ip)

The following variant will iterate over all IP addresses in this network except the broadcast IP address 192.168.1.255 and the network IP address 192.168.1.0:

import ipaddress

network = ipaddress.ip_network('192.168.1.0/24')

for ip in network:
    # Ignore e.g. 192.168.1.0 and 192.168.1.255
    if ip == network.broadcast_address or ip == network.network_address:
        continue
    print(ip)

Problem:

While trying to install a package using pip, you see the following error message:

x86_64-linux-gnu-gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/usr/local/include -I/usr/include -I/usr/include/python3.10 -c src/pyaudio/device_api.c -o build/temp.linux-x86_64-3.10/src/pyaudio/device_api.o
src/pyaudio/device_api.c:9:10: fatal error: portaudio.h: No such file or directory
    9 | #include "portaudio.h"
      |          ^~~~~~~~~~~~~
compilation terminated.
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1
[end of output]

Solution:

On Ubuntu, you need to install the portaudio19-dev package:

sudo apt -y install portaudio19-dev

Problem:

When trying to run your unit tests using tox you see the following error message:

Traceback (most recent call last):
  File "/home/uli/.local/bin/tox", line 5, in <module>
    from tox import cmdline
ImportError: cannot import name 'cmdline' from 'tox' (/usr/local/lib/python3.10/dist-packages/tox/__init__.py)

Solution:

You’re using an older version of tox as an executable. To find out which executable you’re using, check this line of the error message:

File "/home/uli/.local/bin/tox", line 5, in <module>

In our case, the executable is

/home/uli/.local/bin/tox

Delete that file!

After that, tox should work fine. If not, update tox using

pip install -U tox

Problem:

While trying to run your Python project, you see an error message like

AttributeError: module 'scipy' has no attribute 'constants'

even though have have already imported scipy:

import scipy

Solution:

You need to explicitly import scipy.constants

import scipy.constants

I generally recommend to import both scipy and scipy.constants in case you use scipy itself elsewhere in your code:

import scipy
import scipy.constants

If you have an URL like

https://logodix.com/logo/1667872.jpg?param=value

and you want to find just the filename extension (.jpg for this example), use the following code:

from urllib.parse import urlsplit
import os.path

url = "https://logodix.com/logo/1667872.jpg?param=value"

path = urlsplit(url).path
extension = os.path.splitext(path)[-1] # e.g. ".jpg"

Problem:

When trying to urlsplit an URL in Python 3:

from urllib.parse import urlparse

path = urlparse.urlsplit(remote_image_url).path

you see the following error message:

AttributeError                            Traceback (most recent call last)
Input In [30], in <cell line: 3>()
      1 from urllib.parse import urlparse
----> 3 path = urlparse.urlsplit(remote_image_url).path
      4 filename = posixpath.basename(path)

AttributeError: 'function' object has no attribute 'urlsplit'

Solution:

The equivalent of urlparse.urlsplit() in Python 3 is urllib.parse.urlsplit().

Therefore, a working code example is

from urllib.parse import urlsplit

path = urlsplit(remote_image_url).path

Problem:

While trying to import urlparse in Python 3 using

import urlparse

you see the following error message:

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Input In [29], in <cell line: 1>()
----> 1 import urlparse

ModuleNotFoundError: No module named 'urlparse'

Solution:

In Python 3, urlparse has been renamed to urllib.urlparse. In order to import it, use this line instead:

from urllib.parse import urlparse

Assuming you have a Python script server.py that you want to auto-reload every time the file changes, use the following script using nodemon:

nodemon -w . --exec python server.py

The -w . tells  nodemon files to watch for changes of all files in the current directory (.)

I generally recommend creating a script start.sh to automatically run this command:

#!/bin/sh
nodemon -w . --exec python server.py

and then make it executable using

chmod a+x start.sh

Now you can run it using

./start.sh

Returning a JSON array in bottle involves setting the response content type to application/json and then manually dumping the array using json.dumps():

#!/usr/bin/env python3
from bottle import route, run, response
import json

@route('/')
def json_array():
    response.content_type = 'application/json'
    return json.dumps(["a", "b", "c"])

run(host='localhost', port=9000)

Note: I do not recommend using entr any more since for me it doesn’t work reliably. I recommend using nodemon instead, see How to auto-restart bottle server when Python file changes using nodemon

Assuming you have a Python script server.py that you want to auto-reload every time the file changes, use the following script using entr:

ls *.py | entr python server.py

The ls *.py tells entr which files to watch for changes.

This script uses a dedicated class MyServer to encapsulate the bottle server appropriately. Compared to our previous post Python bottle minimal example it provides better encapsulation of the server at the cost of slightly more lines of code.

#!/usr/bin/env python3
from bottle import Bottle, run

class MyServer(object):
    def __init__(self):
        self.app = Bottle()
        self.init_routes()
        
    def init_routes(self):
        """Initialize all routes"""
        @self.app.route('/hello')
        def hello():
            return "Hello World!"

    def run(self):
        run(self.app, host='0.0.0.0', port=8080)
        
# Example usage
if __name__ == "__main__":
    server = MyServer()
    server.run()

How to use

Run the server

python bottle-server.py

and open

http://localhost:8080/hello

Now you should see the Hello World! message.

PyInvoke provides a much easier interface to running CLI processes than subprocess.