[Tutor] still breaking chains

[Steven D'Aprano]

On Sun, Apr 12, 2015 at 11:03:07AM -0700, Jim Mooney wrote:

> If join returns a string, why am I getting a syntax error when I try  to
> slice it?

Because you have a syntax error. Syntax error means you have written 
something which the Python compiler cannot understand, because the 
syntax is wrong, mangled or confused, or something is missing from the 
line of code. (If the missing element is a closing bracket, brace or 
parethesis, the compiler may not report the error until the line *after* 
the offending line.) An English example:

"I would like a cup of coffee" -- correct syntax
"would coffee I cup like"  -- syntax error

Human beings are more flexible with syntax and can sometimes decipher 
rather mangled invalid sentences (e.g. most of us can understand Yoda) 
but programming language compilers are less flexible and require you to 
be pedantically correct:


+ 1 2   # syntax error
function(x y)  # syntax error
'-'.join['a', 'b')  # syntax error


This does not mean that Python cannot add numbers, call functions, or 
join substrings. It means the compiler cannot understand my mangled 
code. Fix the syntax:

1 + 2
function(x, y)
'-'.join(['a', 'b'])


In your example:

> >>> 'alfabeta'[2:5]
> 'fab'
> >>> ''.join(['a', 'l', 'f', 'a', 'b', 'e', 't', 'a')[2:5]
> SyntaxError: invalid syntax

I believe you have deleted what you might have thought was a blank line, 
but actually gives you a hint as to what the error is. When I try to run 
that, I get this:

py> ''.join(['a', 'l', 'f', 'a', 'b', 'e', 't', 'a')[2:5]
  File "<stdin>", line 1
    ''.join(['a', 'l', 'f', 'a', 'b', 'e', 't', 'a')[2:5]
                                                   ^
SyntaxError: invalid syntax


Look carefully at the "blank" line: it isn't actually blank. Way over on 
the right hand side you will see a caret ^ pointing to the spot where 
the compiler first noticed something it could not understand. In your 
email, it may not line up, but in the interactive interpreter the caret 
will line up with ) the closing round bracket. Why? Because before you 
close the method call ''.join( ) you have to close the list [ ]:

    ''.join([...])  # okay
    ''.join([...)  # syntax error

It is sad that the Python compiler does not report a more useful error 
message here. It would be awesome if it would report:

SyntaxError: missing closing bracket ]

but we should count our blessing that we get anything at all.



-- 
Steve