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.

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!

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 operate on

The Range of Content on Planet Python

I've gotten a number of requests lately to contribute only Python related material to the Planet Python feeds and to be honest these requests have both surprised and insulted me, but they've continued. I am pretty sure they've come from a very small number of people, but they have become consistent. This is probably because of my current habit of writing about NaNoWriMo every day and those who aren't interested not looking forward to having the rest of the month reading about my novel. Planet Python will be getting a feed of only relevant posts in the future, but I'm going to be honest: I am kind of upset about it. I don't care if anyone thinks it is unreasonable of me to be upset about it, because the truth is Planet Python means something to me. It was probably the first thing I did that I considered "being part of the community" when I submitted my meager RSS feed to be added some seven years ago. My blog and my name on the list of authors at Plan

Pythonic Defined

Introduction Losing is Good Strings Dictionaries Conclusion Introduction Veterans and novices alike of Python will hear the term "pythonic" thrown around, and even a number of the veterans don't know what it means. There are times I do not know what it means, but that doesn't mean I can define a pretty good idea of what "pythonic" really means. Now, it has been defined at times as being whatever the BDFL decides, but we'll pull that out of the picture. I want to talk about what the word means for us today, and how it applied to what we do in the real world. Languages have their strengths and their idioms (ways of doing things), and when you exploit those you embrace the heart of that language. You can often tell when a programmer writing in one language is actually more comfortable with another, because the code they right is telltale of the other language. Java developers are notorious for writing Java in every language they get their hands on. Ho