如何在Python中获取字符串的子字符串？

Is there a way to substring a string in Python, to get a new string from the third character to the end of the string?

Maybe like myString[2:end]?

If leaving the second part means ’till the end’, and if you leave the first part, does it start from the start?

>>> x = "Hello World!"
>>> x[2:]
'llo World!'
>>> x[:2]
'He'
>>> x[:-2]
'Hello Worl'
>>> x[-2:]
'd!'
>>> x[2:-2]
'llo Worl'

Python calls this concept “slicing” and it works on more than just strings. Take a look here for a comprehensive introduction.

Just for completeness as nobody else has mentioned it. The third parameter to an array slice is a step. So reversing a string is as simple as:

some_string[::-1]

Or selecting alternate characters would be:

"H-e-l-l-o- -W-o-r-l-d"[::2] # outputs "Hello World"

The ability to step forwards and backwards through the string maintains consistency with being able to array slice from the start or end.

Substr() normally (i.e. PHP and Perl) works this way:

s = Substr(s, beginning, LENGTH)

So the parameters are beginning and LENGTH.

But Python’s behaviour is different; it expects beginning and one after END (!). This is difficult to spot by beginners. So the correct replacement for Substr(s, beginning, LENGTH) is

s = s[ beginning : beginning + LENGTH]

A common way to achieve this is by string slicing.

MyString[a:b] gives you a substring from index a to (b – 1).

One example seems to be missing here: full (shallow) copy.

>>> x = "Hello World!"
>>> x
'Hello World!'
>>> x[:]
'Hello World!'
>>> x==x[:]
True
>>>

This is a common idiom for creating a copy of sequence types (not of interned strings), [:]. Shallow copies a list, see Python list slice syntax used for no obvious reason.

Is there a way to substring a string in Python, to get a new string from the 3rd character to the end of the string?

Maybe like myString[2:end]?

Yes, this actually works if you assign, or bind, the name,end, to constant singleton, None:

>>> end = None
>>> myString = '1234567890'
>>> myString[2:end]
'34567890'

Slice notation has 3 important arguments:

• start
• stop
• step

Their defaults when not given are None – but we can pass them explicitly:

>>> stop = step = None
>>> start = 2
>>> myString[start:stop:step]
'34567890'

If leaving the second part means ’till the end’, if you leave the first part, does it start from the start?

Yes, for example:

>>> start = None
>>> stop = 2
>>> myString[start:stop:step]
'12'

Note that we include start in the slice, but we only go up to, and not including, stop.

When step is None, by default the slice uses 1 for the step. If you step with a negative integer, Python is smart enough to go from the end to the beginning.

>>> myString[::-1]
'0987654321'

I explain slice notation in great detail in my answer to Explain slice notation Question.

You’ve got it right there except for “end”. It’s called slice notation. Your example should read:

new_sub_string = myString[2:]

If you leave out the second parameter it is implicitly the end of the string.

I would like to add two points to the discussion:

1. You can use None instead on an empty space to specify “from the start” or “to the end”:

   'abcde'[2:None] == 'abcde'[2:] == 'cde'
   

   This is particularly helpful in functions, where you can’t provide an empty space as an argument:

   def substring(s, start, end):
       """Remove `start` characters from the beginning and `end` 
       characters from the end of string `s`.
   
       Examples
       --------
       >>> substring('abcde', 0, 3)
       'abc'
       >>> substring('abcde', 1, None)
       'bcde'
       """
       return s[start:end]
   

2. Python has slice objects:

   idx = slice(2, None)
   'abcde'[idx] == 'abcde'[2:] == 'cde'
   

If myString contains an account number that begins at offset 6 and has length 9, then you can extract the account number this way: acct = myString[6:][:9].

If the OP accepts that, they might want to try, in an experimental fashion,

myString[2:][:999999]

It works – no error is raised, and no default ‘string padding’ occurs.

Maybe I missed it, but I couldn’t find a complete answer on this page to the original question(s) because variables are not further discussed here. So I had to go on searching.

Since I’m not yet allowed to comment, let me add my conclusion here. I’m sure I was not the only one interested in it when accessing this page:

 >>>myString = 'Hello World'
 >>>end = 5

 >>>myString[2:end]
 'llo'

If you leave the first part, you get

 >>>myString[:end]
 'Hello' 

And if you left the : in the middle as well you got the simplest substring, which would be the 5th character (count starting with 0, so it’s the blank in this case):

 >>>myString[end]
 ' '

Well, I got a situation where I needed to translate a PHP script to Python, and it had many usages of substr(string, beginning, LENGTH).
If I chose Python’s string[beginning:end] I’d have to calculate a lot of end indexes, so the easier way was to use string[beginning:][:length], it saved me a lot of trouble.

Using hardcoded indexes itself can be a mess.

In order to avoid that, Python offers a built-in object slice().

string = "my company has 1000$ on profit, but I lost 500$ gambling."

If we want to know how many money I got left.

Normal solution:

final = int(string[15:19]) - int(string[43:46])
print(final)
>>>500

Using slices:

EARNINGS = slice(15, 19)
LOSSES = slice(43, 46)
final = int(string[EARNINGS]) - int(string[LOSSES])
print(final)
>>>500

Using slice you gain readability.