## Literate programming in python with org-mode and noweb

| categories: | tags: | View Comments

This post examines a different approach to literate programming with org-mode that uses noweb . I have adapted an example from http://home.fnal.gov/~neilsen/notebook/orgExamples/org-examples.html which has some pretty cool ideas in it.

The gist of using noweb is that in your source blocks you have labels like <<imports>>, that refer to other named code blocks that get substituted in place of the label. In the example below, we put labels for a code block of imports, for a function definition, a class definition, and a main function. This code block will get tangled to main.py . The noweb expansion happens at export, so here is the literal code block:

#+BEGIN_SRC python :noweb yes :tangle main.py
<<imports>>

<<some-func>>

<<class-dfn>>

<<main-func>>

if __name__ == '__main__':
status = main()
sys.exit(status)
#+END_SRC


You may want to just check out the org-mode source link at the bottom of the post to see all the details.

import sys
import numpy as np
import matplotlib.pyplot as plt

from argparse import ArgumentParser

def utility_func(arg=None):
return 'you called a utility function with this arg: {0}'.format(arg)

class HelloWorld(object):
def __init__(self, who):
self.who = who

def __call__(self):
return 'Hello {0}'.format(self.who)

def test(self):
return True

def main():
parser = ArgumentParser(description="Say hi")
type=str,
default="world",
help="Who to say hello to")
args = parser.parse_args()

who = args.who

greeter = HelloWorld(who)
greeter()

print 'test func = ', greeter.test()

print utility_func()
print utility_func(5)

return 0

if __name__ == '__main__':
status = main()
sys.exit(status)


## 1 imports

Now, we define a block that gives us the imports. We do not have to use any tangle headers here because noweb will put it in where it belongs.

import sys
import numpy as np
import matplotlib.pyplot as plt

from argparse import ArgumentParser


## 2 utility function

Now we define a function we will want imported from the main file.

def utility_func(arg=None):
return 'you called a utility function with this arg: {0}'.format(arg)


## 3 class definition

Finally, let us define a class. Note we use noweb here too, and we get the indentation correct!

class HelloWorld(object):
def __init__(self, who):
self.who = who

def __call__(self):
return 'Hello {0}'.format(self.who)

def test(self):
return True


### 3.1 some class function

Now, let us make the some-other-func. This block is not indented, but with the noweb syntax above, it seems to get correctly indented. Amazing.

def test(self):
return True


## 4 The main function

This is a typical function that could be used to make your module into a script, and is only run when the module is used as a script..

def main():
parser = ArgumentParser(description="Say hi")
type=str,
default="world",
help="Who to say hello to")
args = parser.parse_args()

who = args.who

greeter = HelloWorld(who)
greeter()

print 'test func = ', greeter.test()

print utility_func()
print utility_func(5)

return 0


## 5 Tangle and run the code

This link will extract the code to main.py:

elisp:org-babel-tangle

We can run the code like this (linux):

python main.py --w John 2>&1
true

test func =  True
you called a utility function with this arg: None
you called a utility function with this arg: 5


or this (windows, which as no sh)

from main import *

main()

test func =  True
you called a utility function with this arg: None
you called a utility function with this arg: 5


## 6 Summary thoughts

The use of noweb syntax is pretty cool. I have not done anything serious with it, but it looks like you could pretty easily create a sophisticated python module this way that is documented in org-mode.