String concatenation without '+' operator

I was playing with python and I realized we don't need to use '+' operator to concatenate strings unless it is used directly.

For example:

    string1 = 'Hello'   'World'  #1 works fine
    string2 = 'Hello' + 'World'  #2 also works fine

    string3 = 'Hello'
    string4 = 'World'
    string5 = string3   string4  #3 causes syntax error
    string6 = string3 + string4  #4 works fine

Now I have two q:

  1. Why statement 3 does not work while statement 1 does?
  2. Is there any technical difference such as calculation speed etc. between statement 1 and 2?

From the docs:

Multiple adjacent string 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".

Statement 3 doesn't work because:

The ‘+’ operator must be used to concatenate string expressions at run time.

Notice that the title of the subheader in the docs is "string literal concatenation" too. This only works for string literals, not other objects.

There's probably no difference. If there is, it's probably extremely tiny and nothing that anyone should worry about.

Also, understand that there can be dangers to this:

    >>> def foo(bar, baz=None):
    ...     return bar
    >>> foo("bob"
    ... "bill")

This is a perfect example of where Errors should never pass silently. What if I wanted "bill" to be the argument baz? I have forgotton a comma, but no error is raised. Instead, concatenation has taken place.