How do I get a substring of a string in Python

Question

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

User · Answer

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.

User · Answer

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

User · Answer

gt  gt  gt  x    Hello World    gt  gt  gt  x 2    llo World    gt  gt  gt  x  2   He   gt  gt  gt  x  -2   Hello Worl   gt  gt  gt  x -2    d    gt  gt  gt  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

User · Answer

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

User · Answer

One example seems to be missing here  full  shallow  copy    gt  gt  gt  x    Hello World    gt  gt  gt  x  Hello World    gt  gt  gt  x     Hello World    gt  gt  gt  x  x    True  gt  gt  gt    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

User · Answer

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

User · Answer

A common way to achieve this is by string slicing    MyString a b  gives you a substring from index a to  b - 1

User · Answer

a  quot Helloo quot  print a  -1    In the above code    -1  declares to print from the starting till the maximum limit-1  OUTPUT    gt  gt  gt  Hello   Note  Here a   -1  is also the same as a  0 -1  and a  0 len a -1   a  quot I Am Siva quot  print a 2     OUTPUT   gt  gt  gt  Am Siva  In the above code a  2   declares to print a from index 2 till the last element   Remember that if you set the maximum limit to print a string  as  x  then it will print the string till  x-1  and also remember that the index of a list or string will always start from 0

User · Answer

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   gt  gt  gt 500   Using slices   EARNINGS   slice 15  19  LOSSES   slice 43  46  final   int string EARNINGS   - int string LOSSES   print final   gt  gt  gt 500   Using slice you gain readability

User · Answer

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

User · Answer

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       gt  gt  gt myString    Hello World    gt  gt  gt end   5    gt  gt  gt myString 2 end    llo    If you leave the first part  you get       gt  gt  gt 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      gt  gt  gt myString end

User · Answer

I would like to add two points to the discussion    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     --------      gt  gt  gt  substring  abcde   0  3       abc       gt  gt  gt  substring  abcde   1  None       bcde              return s start end   Python has slice objects   idx   slice 2  None   abcde  idx      abcde  2       cde

User · Answer

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

[python] How do I get a substring of a string in Python?

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