xtodo python

• Python: Regex Reference

Math of Ordering

In general, the function f used to determine the order of any two element must satisfy some constraints:

• f(a,a)==0
• if f(a,b)==0 then f(b,a)==0
• if f(a,b)==0 and f(b,c)==0, then f(a,c)==0.
• if f(a,b)==-1 and f(b,c)==-1, then f(a,c)==-1.
• if f(a,b)==-1, then f(b,a)==1.

If the comparison function does not behave as the above, then it is not consistent. It means that the result ordered list may actually be different depending the order of the original list or how the language happens to implement sort.

The significance of all this is that in real software you may want to sort a list of complex object by a specialized ordering. e.g. you may want to sort a list of triangles in 3D space by their orientation. Or, sorting image selections in a photo-editing program. It is in advanced cases like these, understanding the basic math about ordering is important. Otherwise, you might have a inconsistent result yet unable to locate any flaws in your code.

The order-deciding function given by the user is called a “predicate” in computer science. (A predicate function is any function that returns either true or false) And, it is usually defined inline using a pure function construct called “lambda”.

• Python, Perl: Split File Path into Parts

dion's try at WolframLang's Map

def map_at_depth(func, node, depth, target_depth, get_children):
    if target_depth < depth:
        return
    else if target_depth == depth:
        yield func(node)
    for child in get_children(node):
        yield from map_at_depth(func, child, depth+1, target_depth, get_children)

WHAT DOES THIS MEANS EXACTLY? :

Empty matches for the pattern are replaced only when not adjacent to a previous match, so "sub('x*', '-', 'abc')" returns '-a-b-c-'.

In addition to character escapes and backreferences as described above, \g<name> will use the substring matched by the group named name, as defined by the (?P<name>…) syntax. \g<number> uses the corresponding group number; \g<2> is therefore equivalent to \2, but isn't ambiguous in a replacement such as \g<2>0. \20 would be interpreted as a reference to group 20, not a reference to group 2 followed by the literal character 0. The backreference \g<0> substitutes in the entire substring matched by the pattern.

• 2022-01-12 blog this python print unicode to stdout sys.stdout.reconfigure(encoding='utf-8')

# python 3

import sys

sys.stdout.reconfigure(encoding="utf-8")
print(sys.stdout.encoding)

# 2022-01-11 need to get python to print unicode and shows as char, when run in emacs
print("unicode • BULLET")