Re: [Tutor] understanding pydoc try
On 08/30/2012 05:15 PM, Alan Gauld wrote: On 30/08/12 15:43, John Maclean wrote: Thanks. This is a heck of a lot more clearer to me! BNF, huh? Another set TLA that I don't need to know ;-) Actually, BNF is one of those useful skills for any programmer because almost every language is 'formally' described using it - at least since the days of Algol, for which it was invented. A simplified version of it is also used to define most command line tools and their arguments so its definitely worth learning, at least the basics. It can save a lot of typing when you want to precisely specify the allowed grammar in a problem. There are tools which can translate BNF like text into something close to code, which is useful if you ever have to define your own programming language. Admittedly not something most programmers ever need to do, but it does happen occasionally that its the easiest way to solve a problem. (The so-called mini-language design pattern) My main issue is that I am a sysadmin and not a programmer. I am aware of pydoc but not of BNF. So I was a bit taken aback when I saw the BNF syntax. It was obvious to me that syntax of the try statements were not python syntax but had no clue how to parse it. BTW - where in pydoc is it mentioned, (or anywhere else for that matter), to refer to BNF? ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] understanding pydoc try
On 08/30/2012 03:05 PM, Dave Angel wrote:
On 08/30/2012 09:30 AM, John Maclean wrote:
What does the first line from `pydoc try` actually mean? This does not
look like the syntax that one is supposed to use.
try_stmt ::= try1_stmt | try2_stmt
You're looking at the first of three BNF statements. BNF (Backus Naur
Form, or something like that) is a way of describing a grammar. i'll
quote the whole thing here, and try to explain it.
The following is from Python 3.2's pydoc:
try_stmt ::= try1_stmt | try2_stmt
try1_stmt ::= "try" ":" suite
("except" [expression ["as" target]] ":" suite)+
["else" ":" suite]
["finally" ":" suite]
try2_stmt ::= "try" ":" suite
"finally" ":" suite
The first statement says that a try_stmt is one or the other of two
formats. This simply says there are two syntaxes you can use, depending
on what try features you want.
The second lists the (most common, i expect) syntax. It has a literal
try token, followed by a literal colon token, followed by a suite of
statements (that's defined elsewhere, but would include simple
statements, if statements, and so on. It wouldn't include def or class,
presumably).
Then there are one or more except clauses. Note the trailing + which
means this element may be repeated, but must be present at least once.
Then there is an optional else clause.
Then an optional finally clause.
These must be present in the specific order stated above. And you can't
(for example) have an else without an except, because except is one or
more times.
The second syntax does not include the except nor else clauses.
Is that clearer?
Thanks. This is a heck of a lot more clearer to me! BNF, huh? Another
set TLA that I don't need to know ;-)
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[Tutor] understanding pydoc try
What does the first line from `pydoc try` actually mean? This does not
look like the syntax that one is supposed to use.
try_stmt ::= try1_stmt | try2_stmt
I can write simple statements as shown below, but I want to actually
understand what I am doing.
try:
import io
print("importing io")
except ImportError:
print("nothing to import")
foo = None
try:
import somefunctionthatdoesnotexist
print("importing ...")
except ImportError:
print("nothing to import")
foo = None
___
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Re: [Tutor] shebang question
>From what I can tell/remember, the first works in the *nix environment if python is in your $PATH, the latter will find python "somehere" on your system by looking at wher the executables should be. On 26/11/06, shawn bright <[EMAIL PROTECTED]> wrote: > Hey there all, > what is the difference between > #!/usr/bin/python > and > #!/usr/bin/env python > ? > > thanks > > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > > > -- John Maclean - 07739 171 531 MSc (DIC) ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
