Skip to main content

PyCon: Core Python Containers--Under the Hood

Core Python Containers--Under the Hood

This was perhaps my favorite talk.

A list is a fixed-length array of pointers.

realloc is called occassionally to grow the list. However, overallocation is used in a very intelligent way to minimize the number of times this is necessary. Rather than simply doubling the size of the list, it's more of a curve. No more than 12.5% of the list is ever empty. The list shrinks when it is half empty.

Memory allocation under Windows is slow.

Inserting to the middle of or shrinking from the middle of a list is O(n). Appending is (on average) O(1).

Use a deque if you want to push or pop from the ends of a list. That's much more efficient than trying to inserting an element at index 0 of a normal list.

Sets are based on fixed-length hash tables. They are kept very sparse. Anytime it becomes 2/3 full, it is grown by a factor of 4. The hash table never needs to be resized for the keyword args dict (assuming you don't modify it).

The builtin set type is faster than implementing your own with dicts because he's able to use some shortcuts when implementing set operations.

On average, there are no more than 1.5 probes per lookup.

Building dicts and sets is expensive.

Dicts are the most finely tuned data structure in the language.

Using a dict in Python is way faster than using a poorly implemented map-like object in C.

Comments

lotrpy said…
jjinuxland, thanks for the info. looking forward to more :)
and, what's the mean of "On average, there are no more than 1.5 probes per lookup." maybe it refers to the set operations?
jjinux said…
It does refer to set operations. I think it has to do with how hashes are implemented. I think it means that hash collisions are low enough that you only need to do on average 1.5 object comparisons when checking for set inclusion. I could be totally wrong ;)

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

Creating Windows 10 Boot Media for a Lenovo Thinkpad T410 Using Only a Mac and a Linux Machine

TL;DR: Giovanni and I struggled trying to get Windows 10 installed on the Lenovo Thinkpad T410. We struggled a lot trying to create the installation media because we only had a Mac and a Linux machine to work with. Everytime we tried to boot the USB thumb drive, it just showed us a blinking cursor. At the end, we finally realized that Windows 10 wasn't supported on this laptop :-/ I've heard that it took Thomas Edison 100 tries to figure out the right material to use as a lightbulb filament. Well, I'm no Thomas Edison, but I thought it might be noteworthy to document our attempts at getting it to boot off a USB thumb drive: Download the ISO. Attempt 1: Use Etcher. Etcher says it doesn't work for Windows. Attempt 2: Use Boot Camp Assistant. It doesn't have that feature anymore. Attempt 3: Use Disk Utility on a Mac. Erase a USB thumb drive: Format: ExFAT Scheme: GUID Partition Map Mount the ISO. Copy everything from