[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 ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] understanding pydoc try
On 30 August 2012 15:30, John Maclean jaye...@gmail.com 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 looks like part of the python language reference. It goes a little further and explains what try1_stmt and try2_stmt actually suppose to mean: http://docs.python.org/reference/compound_stmts.html#the-try-statement 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 Let me try to rephrase it: a try statement is either of try-except-else-finally or of try-finally form. This notation is used to formally describe language syntax: http://docs.python.org/reference/introduction.html#notation best, Michael ___ 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 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? -- DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] understanding pydoc try
On 30/08/12 23:30, 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 That's a description of the Python grammar in some variation of Backus-Naur Form. In English, it means: A try statement is either a try1 statement or a try2 statement. Presumably then there will be a definition of try1_stmt and try2_stmt, also in BNF form, probably in terms of other statements, until eventually the syntax of try statements is fully defined. http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form#Example http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form 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 This first block attempts to import the io module, and then print importing io. If the import process fails, an exception is raised. But since the io module does exist and can be imported, nothing happens and execution happily goes on to the part *after* the except clause: try: import somefunctionthatdoesnotexist print(importing ...) except ImportError: print(nothing to import) foo = None This time, since somefunction blah blah probably doesn't exist, the import process does fail, and an exception is raised, interrupting normal execution of the code. Instead of the next line running, execution of your code halts and Python signals an ImportError. However, then the except ImportError line takes over and catches the exception. nothing to import is printed and foo is set to None. Does that help? -- Steven ___ 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 ;-) ___ 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 10:43 AM, John Maclean wrote: On 08/30/2012 03:05 PM, Dave Angel wrote: snip Thanks. This is a heck of a lot more clearer to me! BNF, huh? Another set TLA that I don't need to know ;-) I learned BNF in about 1972. I've used about 35 languages since (not counting hobby ones). It can clarify a new language better than many paragraphs of description. But I've found that it's seldom completely rigorous. -- DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] understanding pydoc try
On 30-Aug-12 08:22, Dave Angel wrote: On 08/30/2012 10:43 AM, John Maclean wrote: On 08/30/2012 03:05 PM, Dave Angel wrote: snip Thanks. This is a heck of a lot more clearer to me! BNF, huh? Another set TLA that I don't need to know ;-) I learned BNF in about 1972. I've used about 35 languages since (not counting hobby ones). It can clarify a new language better than many paragraphs of description. But I've found that it's seldom completely rigorous. True, usually because people aren't as careful writing it as they are real code that needs to be executed by something. Maybe it would help to start by describing your grammar to YACC, getting it to work, and then expressing that back out as BNF (or just leaving it in YACC code). -- Steve Willoughby / st...@alchemy.com A ship in harbor is safe, but that is not what ships are built for. PGP Fingerprint 4615 3CCE 0F29 AE6C 8FF4 CA01 73FE 997A 765D 696C ___ 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 11:26 AM, Steve Willoughby wrote: On 30-Aug-12 08:22, Dave Angel wrote: On 08/30/2012 10:43 AM, John Maclean wrote: On 08/30/2012 03:05 PM, Dave Angel wrote: snip Thanks. This is a heck of a lot more clearer to me! BNF, huh? Another set TLA that I don't need to know ;-) I learned BNF in about 1972. I've used about 35 languages since (not counting hobby ones). It can clarify a new language better than many paragraphs of description. But I've found that it's seldom completely rigorous. True, usually because people aren't as careful writing it as they are real code that needs to be executed by something. Maybe it would help to start by describing your grammar to YACC, getting it to work, and then expressing that back out as BNF (or just leaving it in YACC code). There's another reason, that I usually assumed to be the case. It usually happens at a place where the grammar is particularly tricky, and where the only valid thing to do in BNF is to list lots of cases (as the one in this thread lists two). So I assumed the BNF was more-or-less deliberately dumbed down to make it more legible. I like your explanation better, though. -- DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] understanding pydoc try
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) -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ 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 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 30/08/12 17:21, John Maclean wrote: 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? If you are writing (or reading!) code you are a programmer! :-) Michael already gave a link to the notation page on the web site which does explicitly mention BNF but, to be honest it would not be surprising if it didn't. It would be like specifically saying that a web page was written in HTML, nowadays its often just assumed that anyone creating web pages knows about HTML... Similarly languages are usually specified in some approximation of BNF. The link was: http://docs.python.org/reference/introduction.html#notation -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
