Why aren't colons optional?

Robert Amesz sheershion at mailexpire.com
Sun Jan 20 19:57:21 CET 2002


Hans Nowak wrote:

> "Edward K. Ream" wrote:
>> 
>> Why are colons required after def, elif, else, except, finally,
>> for, if, try and while?  One would think the colon would be
>> optional when the colon is followed by a newline. 
>> 
>> Obviously, colons are required in: ^
>> 
>>   if a == 1 : b = c
>>   elif a == 2 : b = d
>>   else : b = e
>> 
>> My question is: why isn't the following valid?
>                 ^
> 
> Note the colons in your own post. :-)  They are not really
> necessary for understanding what you wrote, but they make
> the sentence easier to read. Same with Python. The colon
> is not necessary for parsing if it's followed by a code
> block, but it increases readability. It's there for
> human readers.

Not _only_ for human readers: some editors/IDEs use trailing colons to 
provide automatic indentation. When so, the 'redundant' colon is 
actually free, in terms of keystrokes.

>From the point of view of parsing a language, simple rules are better: 
parsing is faster and when things go wrong it's easier to come up with 
helpful error message (although Python still could use some improvement 
in that area).

A particularly horrible example of how *not* to design a language 
syntax comes from EcmaScript (formerly: JavaScript, and before that: 
LiveScript). As with Java and C, EcmaScript requires that each 
statement is terminated by a semicolon, but with a curious exception: 
if the semicolon would have been the last character on the line, and 
some other conditions are met, the semicolon may be omitted.

What a horrible inconsistency! Worse still, newlines aren't just 
whitespace anymore (which is discarded by the lexer) but have become 
tokens in their own right. Most of the time those newline-tokens are 
meaningless garbage in the token stream, but the grammar still has to 
allow for them. What a horrible, horrible mess! And imagine what this 
does for the accuracy and helpfulness of any error message, which is 
always a concern with parsers.

And all that for... Yes, for what, exactly? To save a few keystrokes? 
To satisfy a few script-kiddies who might have started out using 
something like Visual Basic and can't adjust to a free-format syntax? 
It just makes no sense whatsoever, so it's probably a 'political' 
decision. Ugh!

So don't complain or question, but *cherish* the consistency of Python. 
It is a great bonus.


Robert Amesz



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