[Python-ideas] explicitation lines in python ? (original) (raw)

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( '{}'.format(htmlline) for line in table) : # main line htmlline : ''.join( '{}'.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 " for x in mylist" often greatly improve readability of python programs, when is not too complex When 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 newlist.append()

I often choose b), but I dislike both solutions :

Renouncing to list comprehension occurs rather often when I write python code

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( '{}'.format(htmlline) for line in table) : # main line htmlline : ''.join( '{}'.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 :

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.

Daniel

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