On 24/12/19 1:48 PM, Tim Daneliuk wrote:
If I do this:
foo = [ "bar", "baz" "slop", "crud" ]
Python silently accepts that and makes the middle term "bazslop".
BUT, if I do this:
foo = [ "bar", "baz" 1, "crud" ]
or this:
foo = [ "bar", 2 1, "crud" ]
The interpreter throws a syntax error.
This is more of an intellectual curiosity than anything else, but why do
strings silently
concatenate like that, while all other case blow up with an error. i.e., What
is about
the language the promotes this behavior. At first blush, it seems
inconsistent, but what
do I know ...
Restricting our conversation to numbers/integers and strings,
"concatenate" only ever seems to apply to string objects.
(there are meanings of "concatenate" for matrices, for example)
For fun, try:
help( str ) # and
help( int )
They both mention __add__(self, value, /) In both cases such is defined
as "self+value" - but remember that the "+" operator is "over-loaded".
Thus (and I'm telling you nothing new here) when dealing with strings we
read "+" as "concatenate"; whereas with integers it is plainly "add".
Two quite different operators/operations!
However, your point involves the fact that whereas:
1 + 2 # 3 is *clearly* addition, and
"a" + "b" # "ab" is *clearly* concatenation
"a" "b" # also evaluates to "ab"
and is thus, concatenation without any explicit infix operator! Just one
cotton-picking minute - what's going on here?
The answer to all your dreams (um, maybe...) lies in "The Python
Language Reference" docs, particularly Section 2 "Lexical Analysis" (the
process of taking "tokens" and making sense of how they go-together!).
<<<
2.4.2. String literal concatenation
Multiple adjacent string or bytes literals (delimited by whitespace),
possibly using different quoting conventions, are allowed, and their
meaning is the same as their concatenation. Thus, "hello" 'world' is
equivalent to "helloworld". This feature can be used to reduce the
number of backslashes needed, to split long strings conveniently across
long lines, or even to add comments to parts of strings, for example:
re.compile("[A-Za-z_]" # letter or underscore
"[A-Za-z0-9_]*" # letter, digit or underscore
)
Note that this feature is defined at the syntactical level, but
implemented at compile time. The ‘+’ operator must be used to
concatenate string expressions at run time. Also note that literal
concatenation can use different quoting styles for each component (even
mixing raw strings and triple quoted strings), and formatted string
literals may be concatenated with plain string literals.
>>>
Thus, the adjacency of two literals explicitly only implies
concatenation for strings. There is no equivalent/similar mention for
numbers.
WebRef: https://docs.python.org/3/reference/lexical_analysis.html
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