为什么每个字符串都包含空字符串？

Question

"ABCDE" has no empty character. But when I type

"" in "ABCDE"

Python interpreter returns True .

Why?

is there any empty character in "ABCDE"? I don't think so.

And I also found when I use these code:

target = ''
src = 'ABCDE'
src.find(target)

it returns 0 instead of -1

Why is this?

Answer 1

For people with background in languages where string objects represented as arrays of characters it may be surprising, but if we try to follow such approach like

string = 'ABCDE'
characters_list = list(string)

then

'' in characters_list

will be False statement.

Empty string probably came from mathematics , where it is a neutral element for binary operation of string concatenation , ie for every string a

a + empty_string == empty_string + a == a

where + is a string concatenation symbol. Then " substringing " can be defined as follows:

• for every strings a , b we say a is substring of b iff exists strings c , d such that

  b == c + a + d

Let's denote a is substring of b as a in b .

With these definitions of empty string and substringing relation can be proved lemma

• empty_string is a substring of any string a :

   a == (definition of empty_string) == empty_string + a == == (definition of empty_string) == empty_string + empty_string + a

  then if we define c = empty_string and d = a :

   a == c + empty_string + d

  and by definition empty_string in a .

Answer 2

Here's a qualitative way to think about it. Consider the following:

>>> "foo"[0:0]
''

Doing a zero-width slice of a string returns '' . So, if you can get '' out of a string, it must be in the string and therefore '' in "foo" must be true.