The modification of strings in Python and the detailed _python

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Recently encountered a topic in the interview, choose to use JavaScript or Python to implement string inversion, I chose Python, and then wrote the Code (wrong):

#!/usr/bin/env python
#-*-coding:utf-8-*-
__author__ = ' Zhanghe '
def reverse (s):
 L = 0
 r = Len (s)- 1 while
 L < r:
  s[l],s[r] = s[r],s[l]
  L = 1
  R-= 1 return
 s

The interviewer then asked two questions:

(1) Can you modify the value of a string like this? "I can answer," "answer the wrong."

(2) The incoming parameter is an address? Or a copy? "I answered, passing the value. Numbers, strings, tuple values (immutable), list and dict references (mutable), "" "Answer the value, you can directly modify the" "answer the wrong, correct is the value, immutable"

Consider the following

Although the string is often used, but has not really studied the problem, so the Internet search information:

Strings in Python are immutable types, which means that elements that change a string need to create a new string.

A string is composed of independent characters and is also a sequence, and the common method of operation of the sequence also applies to strings.

For example:

Accessing substrings sequentially by slicing operations;

The length of the string is obtained by Len ().

Determines whether a character exists in a string by using the in or not in operator.

Python does not have the type of characters, but instead uses a string of length 1 to represent the concept, which is, of course, a substring.

Access String Examples:

1 astring = ' Hello world! '
2 print (astring[0])
3 print (Astring[1:5])
4 print (astring[6:])

Output:

H
Ello
world!

So how do you change a string?

You can "update" an existing string by assigning it to a variable (or by assigning a value). The new value may be similar to the original value, or it may be completely different from the original string.

For example:

1 astring = ' Hello world! '
2 astring = Astring[:6] + ' python! '
3 Print (astring)
4 astring = ' different string altogether '
5 print (astring)

Output:

Hello python!
Different string altogether

So how do you delete a character or string?

Again, the string is immutable, so you can't just delete a character from a string, you can either empty an empty string, or combine strings that strip out unwanted parts to form a new string.

Suppose you want to get from "Hello world!" Inside to delete lowercase "l", then you need to do this:

1 astring = ' Hello world! '
2 astring = Astring[:3] + astring[4:]
3 print (astring)

Output:

Helo world!

Empty or delete a string by assigning an empty string or by using the DEL statement. However, in most applications, it is not necessary to explicitly delete a string. The code that defines this string ends when Python automatically releases the strings.

So, the inverse string code I wrote is problematic, and the correct code should be:

#!/usr/bin/env python
#-*-coding:utf-8-*-
__author__ = ' Zhanghe '
def reverse (s):
 t = '
 r = Len (s)-1 while
 r>=0:
  t = t + s[r]
  R-= 1 return
 t
s = ' ABCD '
print reverse (s)

So is the incoming formal parameter s and the argument s the same object? You can use the ID function to verify, first look at the official explanation of the ID function.

That is, the id(obj) function returns the address of the object obj in memory during its lifecycle, and the parameter type of the ID function is an object (all objects in Python, references to objects in a variable)

We can use the code below to verify:

#!/usr/bin/env python
#-*-coding:utf-8-*-
__author__ = ' Zhanghe '
def reverse (s):
 print ID (s)
 t = '
 r = Len (s)-1 while
 r>=0:
  t = t + s[r] r-=
  1 return
 t
s = ' ABASDFASDFCDABASDFASDFCD ' 
   print ID (s)
print reverse (s)

Output:

38264224
38264224
Dcfdsafdsabadcfdsafdsaba

You can see that the incoming parameter is actually the address of the string object, and if you change the argument to list or dict, the output ID is the same, so the way to get the parameters in Python is the address of the incoming object, except that numbers, strings and tuple are not directly modified, and the list and dict can be directly modified.

Summarize

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