python

virtual environments

Always work in a virtual environment

create one by;

mkdir virtualenv cd virtualenv virtualenv rates virtualenv -p python3 rates_py3 (for python 3) or python3 -m venv rates_py3 . rates_py3/bin/activate to activate deactivate to deactivate environment

virtualenvwrapper

This is a better wrapper for working within virtualk environments. Configure it;

# install
sudo pip install virtualenvwrapper

# find the shell script and add it to your profile (or zshrc) file
which virtualenvwrapper.sh

# add these lines to .profile or .zshrc
source /usr/local/bin/virtualenvwrapper.sh # path to script

export WORKON_HOME="/Users/kristian/virtualenvs" # this is the location of all virtual environments. otherwise the default .virtualenvironments is used

export PROJECT_HOME="/Users/kristian/python" # this is the location of python projects

Listing environments

$ workon
flask
myenv
new_project

Swapping to new project

workon new_project

Leave enviroment

deactivate

Create new project and virtual environment and bind them

mkproject new_project mkproject -p python3 new_project

To bind existing projects, activate the environment and then run

setvirtualenvproject

Create a virtual environment

mkvirtualenv new_env

Remove. virtual environment

rmvirtualenv new_env

python requirements

Once inside a virtual environment, you can show what packages are installed in this environment by using the freeze command with pip

(myenv) $ python -m pip freeze
certifi==2020.4.5.1
chardet==3.0.4
idna==2.9
python-box==4.2.2
requests==2.23.0
ruamel.yaml==0.16.10
ruamel.yaml.clib==0.2.0
toml==0.10.0
urllib3==1.25.9

you can then easily create a requirements file by running the following command;

(myenv) $ python -m pip freeze > requirements.txt

and install those requirements in a new environment via;

(newenv) $ python -m pip install -r requirements.txt

lists

• list.append(newlist) - will append the values as a single value to an existing list. eg list[1,2,3,[10,20]]
• list.extend(newlist) - adds each value of the newlist to the list. eg list[1,2,3,10,20]

working with files

use ‘with’. this handles opening and closing.

with open(fname, "r+") as fhandle:
  lines = fhandle.readlines(); # reads all lines into a list

or to iterate over each line..

with open('53.up', 'r+') as fhandle:
  for line in fhandle:

beware some methods (such as tell()) do not work within with()

iterate over dictionary (assoc. array)

given an associative array (array), loop over the key, value pairs using items() method;

>>> dict = {"name":"super dude", "age":"300"};
>>> for key,value in dict.items():
...  print ("Key: %s Value: %s" %(key, value));
...
Key: name Value: super dude
Key: age Value: 300

same can be done for keys() or values() methods. Note that keys of hash arrays can be jumbled

equivalent of a for..next loop

range can be used to provide a list of items, after being provided a number (from len for example). range(10) does 0-9, so great for ‘for next’ loops.

>>> for i in range(10):
...  print ("I:",i);
...
I: 0
... (snip)
I: 8
I: 9
>>>

range

provide another number to give a start and end point

>>> for i in range(1,11):
...  print ("I:",i);
...
I: 1
I: 2
..snip
I: 7
I: 8
I: 9
I: 10

provide a 3rd number for step value.

>>> for i in range(1,11,2):
...  print ("I:",i);
...
I: 1
I: 3
I: 5
I: 7
I: 9
>>>

here we loop over a list

>>> items = ['one', 'two', 'three'];
>>> for index in range(len(items)):
...  print ('Current item:', items[index]);
...
Current item: one
Current item: two
Current item: three

notify_slack

import requests

# handler for slack notifications
# create a webhook_url in slack and use below
# also update the CHANNELNAME to reflect your own
def notify_slack(msg, icon = ':starfleet:'):

  logging.debug('Notifying Slack with results')

  webhook_url = 'https://hooks.slack.com/services/AAAAAAA/BBBBBBB/WEBHOOKTOKEN'
  payload = {"channel": "#CHANNELNAME", "username": "webhookbot", "text": msg, "icon_emoji": icon}

  response = requests.post(
    webhook_url, data=json.dumps(payload),
    headers={'Content-Type': 'application/json'}
  )
  if response.status_code != 200:
    raise ValueError(
        'Request to slack returned an error %s, the response is:\n%s'
        % (response.status_code, response.text)
    )