Using python 2 and 3 for development¶

Install python 3.5 and 2.7.11¶

Then use virtualenv -p to do ~/ENV2 and ~/ENV3 create alias in bashrc for ENV2 and ENV3

OSX¶

We assume that you have downloaded and installed both python packages for 2.7.11 and 3.5 while getting them from

http://python.org

Make sure you have both versions installed:

$ ls /Library/Frameworks/Python.framework/Versions/3.5/bin
$ ls /Library/Frameworks/Python.framework/Versions/2.7/bin

Next you may run outdated versions of pip that you need to update and install virtualenv:

.. prompt:: bash

pip3.5 install pip -U pip3.5 install –user virtualenv pip2.7 install pip -U pip2.7 install –user virtualenv

Edit the bash_profile with your prefered editor and append some useful aliasses:

emacs ~/.bash_profile

Append the following aliasses:

alias virtualenv3='~/Library/Python/3.5/bin/virtualenv'
alias virtualenv2='~/Library/Python/2.7/bin/virtualenv'
alias ENV2=`source ~/ENV2/bin/activate`
alias ENV3=`source ~/ENV3/bin/activate`

Next source the bashrc file so you have accesss to the new commands

source ~/.bash_profile

Next let us create two virtualenv’s one for python 2, the other for python 3 in the directories ~/ENV2 and ~/ENV3.

virtualenv3 ~/ENV3
virtualenv2 ~/ENV2

Now we can use the ENV2 and ENV3 commands to activate selectively which version of python we use:

ENV2
python --version

ENV3
python --version

The python versions should be 2.7.11 or 3.5.1 or greater. To do a check on pip, say

pip --version

The pip version should be 8.0.2 or greater

Python Guidelines¶

Exceptions¶

from cloudmesh_client.common.Error import Error
Error.msg(“msg")
Error.info(“msg”)
Error.debug(“msg”)
Error.warning(“msg”)
Error.exit(“msg)


try:
   ...
except:
   Error.debug(“my debug msg”)

Developing Compatible Code¶

A good resource is the Python 2 and 3 compatibility cheat sheet:

http://python-future.org/compatible_idioms.html

We have adopted the following

print¶

from __future__ import print_function

print('Hello')

raise¶

raise ValueError("dodgy value")

traceback¶

from future.utils import raise_

traceback = sys.exc_info()[2]
raise_(ValueError, "dodgy value", traceback)

# we will need to create function for that as we want to control
# with flag in db

exception¶

except Exception as e:

integer division¶

from __future__ import division

assert 2 / 3 == 0

metaclass¶

from future.utils import with_metaclass

class Form(with_metaclass(FormType, BaseForm)):
    pass

strings¶

from __future__ import unicode_literals

s1 = 'The Zen of Python'

import¶

all files will need

from __future__ import absolute_import
from __future__ import print_function
from __future__ import unicode_literals

Dict¶

Using dict heights = {‘Fred’: 175, ‘Anne’: 166, ‘Joe’: 192} as an example

Iterable dict keys:

for key in heights:
    ...

Iterable dict values:

from builtins import itervalues
for key in itervalues(heights):
    ...

Iterable dict items:

from future.utils import iteritems
for (key, value) in iteritems(heights):
    ...

dict keys as a list:

keylist = heights.keys() NO!
keylist = list(heights) YES!

dict values as a list:

from future.utils import itervalues
valuelist = list(itervalues(heights))

dict items as a list:

from future.utils import iteritems
itemlist = list(iteritems(heights))

dict comparison not supported any more!:

a = {"key":"value1"}
b = {"key":"value2"}
if a > b:     # NOT WORKING!!!
    DO SOMETHING!

File¶

Open file to read:

f = file(pathname) NO!
f = open(pathname) YES!

Reading the file into an array

with open(filename, "r") as f:
   content = f.read()
content = content.split("\n")

input¶

Getting raw input from keyboard:

name = raw_input('What is your name? ') NO!

from builtins import input
name = input('What is your name? ')
YES!

bytechar¶

from builtins import bytes

for myint in bytes(b'byte-string with high-bit chars like'):
    bytechar = bytes([myint])