So 'in' is a comparison "operator", is it? I am annoyed at how long it took me to verify that Python treats it as such, and I am also annoyed that it is so.
http://docs.python.org/tutorial/datastructures.html 5.7. More on Conditions¶ The conditions used in while and if statements can contain any operators, not just comparisons. The comparison operators in and not in check whether a value occurs (does not occur) in a sequence. I never cared for the misleading a<b<c notation anyway, and won't use it. On Tue, Mar 29, 2011 at 19:09, Carl Cerecke <c...@free.org.nz> wrote: > My experience of confusing boolean expressions was the code: > >>>> 1 == 2 in [2, False] > False > > which, when parenthesised the two possible ways >>>> (1 == 2) in [2, False] > True >>>> 1 == (2 in [2, False]) > True > results in sensible values (although the second one's sensibility is > debatable) > > I couldn't figure it out quickly either. In fact, I ended up decompiling to > byte code to figure out what was going on. > > As far as other 'gotcha's in addition to the ones already mentioned: > > Calling a file the same name as a module in the standard library. Import > <name> will get the local file, not the stdlib one, which is confusing if > you wanted the stdlib one. > > I've noticed that students will somtimes put import statements inside a > function - right before they need to use whatever they imported: > def area(r): > import math > return math.pi*r*r > > Cheers, > Carl. > > On 29 March 2011 18:06, Michael H. Goldwasser <goldw...@slu.edu> wrote: >> >> To start a new thread, I'm always trying to keep a list of some common >> "gotchas" that beginning students run across when using Python, or >> things that teachers should keep in mind when teaching with the >> language. I have in mind commands that do not generate runtime errors, >> but are likely to lead to logical errors that are not apparent to >> students, and most of these are legal due to the very flexible nature >> of the Python language. Some classicss are >> >> data.sort # no-op >> >> data = data.sort() # bye-bye data >> >> result = result.upper # probably not what you expect >> >> if answer.isupper: # always true ("but I entered a lowercase >> answer") >> >> >> This semester, I ran across a new one that took me quite some time to >> figure out what was happening. This started with a habit that too >> many students have of wanting to compare boolean values to True/False >> literals, rather than just using the boolean as a condition. That is, >> students seem drawn to the syntax >> >> if answer.isupper() == True: >> >> rather than the preferred >> >> if answer.isupper(): >> >> These complaints are more of style than substance, as the two >> versions are logically equivalent. But then a student came to me >> with code that wasn't behaving. The syntax they used was something >> akin to >> >> >> if x > y == True: >> >> however the condition was not being triggered, even when we verified >> that x was indeed greater than y. You can try this out with explicit >> values as follows. >> >> >>> if 5 > 4 == True: >> ... print "Good" >> >>> >> >> You will find that the body is not executed. More directly, you can >> look at the conditional value directly as >> >> >>> 5 > 4 >> True >> >>> 5 > 4 == True >> False >> >> This baffled me for an hour before finally seeing what was happening. >> I'll leave this as a puzzle for readers (many of whom I'm sure will be >> quicker than I was to see the solution). For those who give up, I >> offer the following link as a spoiler. >> http://docs.python.org/reference/expressions.html#comparisons >> >> With regard, >> Michael >> >> +----------------------------------------------- >> | Michael H. Goldwasser, Ph.D. >> | Associate Professor >> | Director of Computer Science >> | Dept. Mathematics and Computer Science >> | Saint Louis University >> | 220 North Grand Blvd. >> | St. Louis, MO 63103-2007 >> | >> | Office: Ritter Hall 108 >> | Email: goldw...@slu.edu >> | URL: http://cs.slu.edu/~goldwasser >> | Phone: (314) 977-7039 >> | Fax: (314) 977-1452 >> >> _______________________________________________ >> Edu-sig mailing list >> Edu-sig@python.org >> http://mail.python.org/mailman/listinfo/edu-sig > > > _______________________________________________ > Edu-sig mailing list > Edu-sig@python.org > http://mail.python.org/mailman/listinfo/edu-sig > > -- Edward Mokurai (默雷/धर्ममेघशब्दगर्ज/دھرممیگھشبدگر ج) Cherlin Silent Thunder is my name, and Children are my nation. The Cosmos is my dwelling place, the Truth my destination. http://www.earthtreasury.org/ _______________________________________________ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
