Skip to main content

How to Confuse _ and locals()["_[1]"]

So, after posting about the exception raising list comprehensions, I got this:
Kevin has left a new comment on your post "How to Add Memory Leaks to Python":

Doesn't '_' only exist in the interactive interpreter?
Kevin has a misunderstanding here in that there is a huge difference between the expressions _ and the locals()["_[1]"]. You might spot why they are so different, or you might not. The second, the one from the list comprehension, is unable to be accessed by name directly. You can only get at it via the locals() and globals() functions, depending on your scope (locals() always works right after the LC in question, though). The name is intentially something that, if tried to resolve as an actual name in the scope, won't find the object in question. Python will look for _ and then do a subscript lookup on key 1 on it. This is completely different than actually looking up the name _[1] in the dictionary where names are stored and grabbing up the value its bound to.

So, Kevin, your question can be answered in that its irrelevant, because we aren't dealing with _ at all. Also, open your profile so I can respond to you directly, next time.
Labels:

Comments

Kevin Dangoor said…
Hey, thanks for the clarification.

I had actually tried it by taking your program and running it through Python directly. I made the mistake of not putting a try: except: around that, so it never got to my print globals()['_[1]'] statement. Oops!

I should've realized that the ['_[1]'] was just an ordinary dictionary lookup and not something weird and magical (though this key's existence is a little weird and magical!)

There are actually quite a few unexpected ways in which Python programs can leak memory. Thanks for bringing up one more!
3:53 PM
Post a Comment

Popular posts from this blog

CARDIAC: The Cardboard Computer

I am just so excited about this. CARDIAC. The Cardboard Computer. How cool is that? This piece of history is amazing and better than that: it is extremely accessible. This fantastic design was built in 1969 by David Hagelbarger at Bell Labs to explain what computers were to those who would otherwise have no exposure to them. Miraculously, the CARDIAC (CARDboard Interactive Aid to Computation) was able to actually function as a slow and rudimentary computer.  One of the most fascinating aspects of this gem is that at the time of its publication the scope it was able to demonstrate was actually useful in explaining what a computer was. Could you imagine trying to explain computers today with anything close to the CARDIAC? It had 100 memory locations and only ten instructions. The memory held signed 3-digit numbers (-999 through 999) and instructions could be encoded such that the first digit was the instruction and the second two digits were the address of memory to operat...
1 comment
Read more

Announcing Feet, a Python Runner

I've been working on a problem that's bugged me for about as long as I've used Python and I want to announce my stab at a solution, finally! I've been working on the problem of "How do i get this little thing I made to my friend so they can try it out?" Python is great. Python is especially a great language to get started in, when you don't know a lot about software development, and probably don't even know a lot about computers in general. Yes, Python has a lot of options for tackling some of these distribution problems for games and apps. Py2EXE was an early option, PyInstaller is very popular now, and PyOxide is an interesting recent entry. These can be great options, but they didn't fit the kind of use case and experience that made sense to me. I'd never really been about to put my finger on it, until earlier this year: Python needs LÖVE . LÖVE, also known as "Love 2D", is a game engine that makes it super easy to build ...
Post a Comment
Read more

My Software Job Transition Strategies?

I’ve been spending a good deal of the last two days preparing mentally for starting a whole new challenge as a developer. New things aren’t new to me, but this is different and big enough really call for some Deep Thoughts ™. For one thing, I’ve made a big move from the world of Python web development to totally other Python work and while web development has never been the only thing I do, it has been the only work that paid the bills. That transition isn’t one that bothers me or daunts me, though. Instead, I’m thinking about transitioning to the scope of the work I’m getting into. For a long time, I juggled multiple clients and client projects every day, so no single project usually took up most of my time. Every developer juggles time through the day, but exactly how that works in each company and on each project varies a lot. I was looking for a place that I could really focus in a way that I haven’t for a long time. I think I found that, but now I have to deal with the consequen...
Post a Comment
Read more