[Python-ideas] explicitation lines in python ?

Daniel DELAY danieldelay at gmail.com
Fri Jun 25 21:08:31 CEST 2010

If we could  explicitate a too complex expression in an indented next 
line, I would use this feature very often :

htmltable = ''.join( '<tr>{}</tr>'.format(htmlline) for line in table) 
:    # main line
     htmlline : ''.join( '<td>{}</td>'.format(cell) for cell in 
line)     # explicitation(s) line(s)

(Sorry if this has already been discussed earlier on this list, I have 
not read all the archives)


in details :

List comprehension  "<expression> for x in mylist"  often greatly 
improve readability of python programs, when <expression> is not too complex
When <expression> is too complex (ex: nested lists), this become not 
readable, so we have to find another solution  :
a) defining a function expression(x), or an iterator function, wich will 
only used once in the code
b) or droping this beautiful syntax to replace it the very basic list 
construction :
newlist = []
for x in myiterable

I often choose b), but I dislike both solutions :
- in solution a) function def can be far from list comprehension, in 
fact instructions to build the new list are split in two different 
places in the code.
- solution b) seems a bit better to me, but the fact we build a new list 
from myiterable is not visible in a glance, unlike list comprehensions.

Renouncing to list comprehension occurs rather often when I write python 

I think we could greatly improve readability if we could keep list 
comprehension anywhere in all cases, but when necessary explicit a too 
complex expression in an indented line :

htmltable = ''.join( '<tr>{}</tr>'.format(htmlline) for line in table) 
:    # main line
     htmlline : ''.join( '<td>{}</td>'.format(cell) for cell in 
line)       # explicitation(s) line(s)

In the case the main line is the header of a "classical" indented block 
(starting with "for", "if", "with"...) , this idented block would simply 
follow the explicitation(s) line(s).
The explicitations lines can be surely identified as the lines than 
begin with "identifier :"  (when we are not in an unclosed dict)

with open('data.txt') as f :
     if line in enumerate(mylist) :  # main line
         mylist : f.read().strip().lower()    # explicitation(s) line(s)
         print line    # "classical" indented block

Another possible use of "explicitations lines" is a coding style wich 
would start by "the whole picture" first, and completing with details 
after, wich is the usual way we mentally solve problems.

Let's take an example : we want to write a function wich returns a 
multiplication table in a simle html document.
When I solve this problem, il think a bit like that :
- I need to return an html page. For that I need a "header" and a 
"body". My body will contain an "htmltable", wich be built from a 
"table" of numbers etc.

My code could look like that :

def tag(content, *tags): # little convenient function
     retval = content
     for t in tags:
         retval =  '<{0}>{1}</{0}>'.format(t, retval)
     return retval

def xhtml_mult_table(a, b):
     return tag(header + body, 'html') :
         header :  tag('multiplication table', 'title')
         body : tag(htmltable, 'tbody', 'table', 'body') :
             htmltable : ''.join(tag(xhtmlline, 'tr') for line in table) :
                 table : headerline + otherlines :
                     headerline : [[''] + range(a)]
                     otherlines : [[y] + [x*y for x in range(a)]  for y 
in range(b)]
                 xhtmlline : ''.join(tag(str(cell), 'td') for cell in line)

This example is a "heavy use" of the "explicitation line" feature, to 
illustrate how it could work.

I don't mean this should replace the "classical" syntax everywhere 
possible, but for me this would be for me a nice way to explicitate 
complex expressions from time to time, and the ability to use list 
comprehension everywhere I wan't.


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