Programming languages

Problem:

You want to inherit a Python class from enum like this:

import enum

class MyEnum(enum):
    X = 1
    Y = 2class

but when you try to run it, you see an error message like this:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    class MyEnum(enum):
TypeError: Error when calling the metaclass bases
    module.__init__() takes at most 2 arguments (3 given)

Solution:

You are not trying to inherit from the Enum class (capital E!) but from the enum module!

This is the correct syntax:

from enum import Enum
class MyEnum(Enum):
    X = 1
    Y = 2

Problem:

You are trying to inherit a class from enum in Python:

class MyEnum(enum):
    X = 1
    Y = 2

But when you try to run it, you see this error message:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-15-ebcfa41a8a7c> in <module>
----> 1 class MyEnum(enum):
      2     X = 1
      3     Y = 2

NameError: name 'enum' is not defined

Solution:

You need to inherit from Enum (capital E!), not from enum! The correct syntax is

from enum import Enum

class MyEnum(Enum):
    X = 1
    Y = 2

Problem:

You have code like this in Python:

from enum import Enum

But when you try to run it, you encounter this error:

Traceback (most recent call last):
  File "test.py", line 1, in <module>
    from enum import Enum
ImportError: No module named enum

Solution:

The enum module is only available in Python 3! You are trying to use it in Python 2.

Try running your code with Python3 (e.g. python3 myscript.py). In case that’s not possible since your project or a library is not compatible with Python 3, you can install enum34 which provides the enum package for Python 2:

sudo pip install enum34

Problem:

You have a line like this in your Python code:

from enum import Enum

But when you try to run it, you see this error message:

Traceback (most recent call last):
  File "test.py", line 1, in <module>
    from enum import Enum
ImportError: No module named enum

Solution:

The enum module is only available in Python 3, but you are using Python 2!

You can try to run your script using Python 3. In case that’s not possible (because your project or a library is not compatible with Python 3), you can install the enum34 backport

sudo pip install enum34

After installing this, the ImportError should disappear.

Note: This example shows how to create a temporary directory that is not automatically deleted. Check out How to create a self-deleting temporary directory in Python for an example on how to create a self-deleting temporary directory!

Minimal example:

import tempfile
tempdir = tempfile.mkdtemp()
print(tempdir) # prints e.g. /tmp/tmpvw0936nd

tempfile.mkdtemp() will automatically create that directory. The directory will not automatically be deleted!

Custom prefix (recommended):

import tempfile
tempdir = tempfile.mkdtemp(prefix="myapplication-")
print(tempdir) # prints e.g. /tmp/myapplication-ztcy6s2w

How to delete the directory

In order to delete the temporary directory including all the files in that directory, use

import shutil
shutil.rmtree(tempdir)

If you use requests or urllib3, requests with SSL verification disabled will print this warning:

/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py:851: InsecureRequestWarning: Unverified HTTPS request is being made. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  InsecureRequestWarning)

If you want to disable this warning and you can’t just enable verification, add this code

import urllib3
urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning)

at the top of your Python file.

If you have code that uses subprocess.check_output() to call a command like

subprocess.check_output("ls .", shell=True)

you can use the cwd=... argument of subprocess.check_output() to define the working directory. Example:

subprocess.check_output("ls .", cwd="../", shell=True)

cwd means change working directory and is interpreted relative to the current working directory. However, you can also use absolute paths:

subprocess.check_output("ls .", cwd="/etc/", shell=True)

To redirect in bottle, use this snippet:

response.status = 303
response.set_header('Location', 'https://techoverflow.net')

303 is the HTTP response code See Other. In certain applications you might want to use 301 (permanent redirect) or 307 (temporary redirect) instead.

Full example:

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

@route('/')
def redirect():
    response.status = 303
    response.set_header('Location', 'https://techoverflow.net')

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

Minimal hello world-like example using the bottle HTTP server:

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

@route('/')
def index():
    return "Hello bottle!"

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

Run this file (and keep it running) and then navigate to http://localhost:9000 to see the Hello bottle! message.

In case you see this error message:

Traceback (most recent call last):
  File "./server.py", line 2, in <module>
    from bottle import route, run
ModuleNotFoundError: No module named 'bottle'

you need to install bottle using

sudo pip3 install bottle

or

sudo pip install bottle

depending on which Python version and configuration you use.