🔒 Visualizing a tree

[lunar jetty]

I'm using a data structure called a trie, which is a type of tree. When I construct a trie its hard to know if I'm doing it correctly without any way to actually print out or view the data structure. How would I go about viewing it for debugging purposes.

[fading hare]

@lunar jetty

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[obsidian storm]

A recursive function that formats it as a branching tree is actually a great practice mini-project.

Like shown on the right here: https://ascii-tree-generator.com/

[lunar jetty]

interesting

i may be able to make something like that

[obsidian storm]

Have you worked with recursion before?

[hardy summit]

For a simpler method regarding implementation: you can just implement pre-order, in-order, and post-order tree traversals and use these to identify what the tree looks like.

Also using a debugger can help immensely

[lunar jetty]

Yeah I have

[lunar jetty]

True but then I’d have to decipher it kind of on my own right

Based on all the traversals

For the recursive approach where I print the tree, I would want pre order right

I guess I’ve never done a traversal of a non binary tree

But it’s probably the same concept right

(Tries are not binary)

[hardy summit]

Yeah, probably there's no really such a thing as an in-order traversal for a trie. But pre-order and post-order should still work fine

[round star]

For me the quick thing (with any tree) is a recursive walk of the tree, passing an "indent" argument, default (and thus initially) 0. Call:

print(" "*indent, ...the-node...)

and then call traverse on each subnode with an increased indent value, +1 or +2 or such.
You can always sort the subnode values for a nice ordering.

@lunar jetty ^^

[lunar jetty]

okay cool thats kinda what i was thinking

instead of just passing the subtree you need to pass the depth as well

[round star]

Yes., that's effective what the indent is.

[lunar jetty]

yup

[hardy summit]

Does print have some prefix argument, smth like end, but for the beginning of the string?

[round star]

No.

[hardy summit]

Meh, ok

[round star]

It only has sep, end and file

[obsidian storm]

end probably only exists because you typically want to end in a newline, but also want that to be overrideable. You typically don't want to prefix something before every print, so there's nothing to override.

[lunar jetty]

ill overload the print funtion and add a prefix argument 😉

[round star]

Very sound.

(I actually do overload print in some of my programmes which has a "live" display, like a status bar and progress bars etc. Undraw them all, do the print, redraw below.)

[lunar jetty]

i dont know how to actually overload it

is it as easy as def print():

[hardy summit]

I tried to do it myself, too. It was one of my first, if not my first, actual small project with Python: https://github.com/SamuelRoettgermann/temporary-print/tree/main

[round star]

Yes. You would usually want to keep the old print for use.

builtin_print = print

def print(......):
    builtin_print(.....)

For added fun you can monkeypatch builtins.print to affect most other prints.

[lunar jetty]

can you not use the built in print function just by using print()? since it has different arguments it wont use the overloaded one?

[round star]

No.

[lunar jetty]

ah why not

[round star]

You've rebound the name print.

So saying print now finds your new function.

There's no "overloading" like you might find in a polymorphic language.

[lunar jetty]

wait can you literally not do that in python

where the same name refers to 2 functions

depending on arguments

[round star]

You have to do the same result by other means.

[hardy summit]

no

[lunar jetty]

wow i didnt know that

[round star]

Yeah. Instead you have to have the function figure that out itself.

Assuming you go that route.

[lunar jetty]

ah so when i overload the addition operator in cpp, i cant do it the same way in python

thats when you use dunder methods?

[round star]

Yeah.

A dunder method is what implements (typically) an operator. FOr a specific class.
So a + b calls type(a).__add__(a,b)

[lunar jetty]

i see

[round star]

So if a references an int, it runs int.__add__

[lunar jetty]

interesitng

like i knew that

but i didnt realize it was replacing the overloading i was used to

but like obviously it is

[round star]

Yeah. Different languages.

A def is really just a fancy assignment statement, rebinding the function name. So is an import.

[hardy summit]

The way Python does that is by giving you the option to specify default arguments, e.g.:

def f(x: int = 4, y: str = "abc")
    ...

# all valid
f(2, 'x')
f(4)
f()

And if you want alternating types... well:

def f(x: int | str | None = None):
    if isinstance(x, int):
        ...
    elif isinstance(x, str):
        ...
    else:
        ...
```That's one way to do it

[lunar jetty]

wowza

but c++ also has default arguments

[round star]

Yes.

But python doesn't have a "switch the function based on the call signature". i.e. no overloading. It's just one function. If it receives different types, handling them is up to that single function.

[hardy summit]

Yeah, but C++ is different, because there a function is uniquely identified by it's name and its parameters. In Python there's just a single variable that's identified by name

[lunar jetty]

because everything in python is an object?

[hardy summit]

Not sure if that's really the "why", but it doesn't sound completely impossible

[lunar jetty]

fair enough

[round star]

I've got an @promote decorator I use for "promoting" specific parameters. Eg:

  @promote
  def url_entity(self, url: URL):
    ''' Return the `SiteEntity` associated with this URL, or `None`
        for an unrecognised URL.

Here an URL is a class for URLs, with various convenient methods and attributes. @promote will turn eg a str into URL so the function receives a URL.
Lets me call it with either a URL object or a string like "https://......"

It effectively does that instance stuff Monke showed earlier, then calls the original function.

[round star]

No, because names just refer to objects (eg a function) and calling a function is based entirely on what the name references. So there's no "multiple functions" to pick.

You could make a wrapper which did that fanout though, returning a single function which examining its arguments and calls other functions.

[hardy summit]

Also in C++ you can't have a variable with multiple possible types.

*That's only partially correct because there is std::variant which allows you to "group" a bunch of types... It'd look like so:

void f(std::variant<int, std::string> *x = nullptr) {
    if (!x) {
        // None case in Python
    } else if (std::holds_alternative<int>(*x)) {
        int value = std::get<int>(*x);
        // ...
    } else if (std::holds_alternative<std::string>(*x)) {
        auto value = std::get<std::string>(*x);
        // ...
    }
}

In that code, the type of x would be unique. Namely the type of x would be a "raw pointer to a std::variant<int, std::string>"

[crisp bison]

or for simple cases, you could use the one that's provided already 😉
https://docs.python.org/3/library/functools.html#functools.singledispatch

multiple dispatch can be thought of as just a combination of single dispatches, but to make that sort of thing work you're probably also getting into a lot of complicated functools.partial etc. work. Still...

[fading hare]

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