[Python-ideas] The non-obvious nature of str.join (was Re: sum(...) limitation)

[Terry Reedy]

On 8/11/2014 1:14 PM, Alexander Belopolsky wrote:

> sep.join(list) is not such a weird construct when sep is non-empty - it
> is the sep='' case which is weird and non-obvious.  (Note that someone
> in this thread suggested demonstrating s == sep.join(s.split(sep))
> invariant as a teaching tool, but this invariant fails when sep is
> empty.)

Because s.split('') raises "ValueError: empty separator".  I expected 
the result should be the same as list(s), making the invariant above 
true.  But perhaps Guido thought splitting on '' might be a bug. 
re.split('', s) returns s, which seems wrong. The doc talks about 
'occurences of the pattern'.  I *see* an occurance of '' at every slice 
point.  In a sense, Python slicing does too.
 >>> 'abc'[1:1]
''

When I first learned Python, I knew to test edge case behavior rather 
than depend on my interpretation of docs, or worse, my expectations from 
previous experience and knowledge.  The interactive interpreter makes 
little tests like 'ab'split('') and re.split('', 'ab') trivial and 
faster than reading (or debugging).

-- 
Terry Jan Reedy