python class variable int vs array

You are correct - in the case of Foo.a accessing self.a actually accesses Foo.a, which is shared between all instances of Foo. However, when you update self.n with += you actually create an instance-level variable on self that shadows Foo.n:

>>>
import dis
   >>>
   dis.dis(Foo.bar)
5 0 LOAD_FAST 0(self)
3 LOAD_ATTR 0(a)
6 LOAD_ATTR 1(append)
9 LOAD_CONST 1('foo')
12 CALL_FUNCTION 1
15 POP_TOP

6 16 LOAD_FAST 0(self)
19 DUP_TOP
20 LOAD_ATTR 2(n)
23 LOAD_CONST 2(1)
26 INPLACE_ADD
27 ROT_TWO
28 STORE_ATTR 2(n)
31 LOAD_CONST 0(None)
34 RETURN_VALUE

Suggestion : 2

ob_digit, which contains the actual integer value that we expect the Python variable to represent.,Let's consider now what happens when we use a Python data structure that holds many Python objects. The standard mutable multi-element container in Python is the list. We can create a list of integers as follows:,Notice the main difference: in C, the data types of each variable are explicitly declared, while in Python the types are dynamically inferred. This means, for example, that we can assign any kind of data to any variable:,Here we've switched the contents of x from an integer to a string. The same thing in C would lead (depending on compiler settings) to a compilation error or other unintented consequences:

/* C code */
int result = 0;
for (int i = 0; i < 100; i++) {
   result += i;
}

# Python code
result = 0
for i in range(100):
   result += i

# Python code
x = 4
x = "four"

/* C code */
int x = 4;
x = "four"; // FAILS

struct _longobject {
   long ob_refcnt;
   PyTypeObject * ob_type;
   size_t ob_size;
   long ob_digit[1];
};

L = list(range(10))
L

Suggestion : 3

The principal built-in types are numerics, sequences, mappings, classes, instances and exceptions.,Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:,There are three basic sequence types: lists, tuples, and range objects. Additional sequence types tailored for processing of binary data and text strings are described in dedicated sections.,Return the number of non-overlapping occurrences of subsequence sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.

>>> n = -37 >>>
   bin(n)
'-0b100101' >>>
n.bit_length()
6

def bit_length(self):
   s = bin(self) # binary representation: bin(-37) -- > '-0b100101'
s = s.lstrip('-0b') # remove leading zeros and minus sign
return len(s) # len('100101') -- > 6

>>> n = 19 >>>
   bin(n)
'0b10011' >>>
n.bit_count()
3
   >>>
   (-n).bit_count()
3

def bit_count(self):
   return bin(self).count("1")

>>> (1024).to_bytes(2, byteorder = 'big')
b '\x04\x00' >>>
   (1024).to_bytes(10, byteorder = 'big')
b '\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00' >>>
   (-1024).to_bytes(10, byteorder = 'big', signed = True)
b '\xff\xff\xff\xff\xff\xff\xff\xff\xfc\x00' >>>
   x = 1000 >>>
   x.to_bytes((x.bit_length() + 7) // 8, byteorder='little')
      b '\xe8\x03'

>>> int.from_bytes(b '\x00\x10', byteorder = 'big')
16
   >>>
   int.from_bytes(b '\x00\x10', byteorder = 'little')
4096
   >>>
   int.from_bytes(b '\xfc\x00', byteorder = 'big', signed = True) -
   1024 >>>
   int.from_bytes(b '\xfc\x00', byteorder = 'big', signed = False)
64512
   >>>
   int.from_bytes([255, 0, 0], byteorder = 'big')
16711680

Suggestion : 4

Pragmatically, casting is most often performed by itself in a variable declaration, as illustrated above. As an example, most classes include an equals method that allows comparison with a reference to any object. For example, the Rational class should include an equals method defined by , The first method finds and returns the lowest index that stores the String value specified by the second parameter. Note the form of the parameter variable for the array: it is the same as declaring a local variable of the String[] type. , Finally, the toString method returns the value of top, the length of the stack array, and the String values of all the references in the stack. It uses lots of catenation to get the job done. , Here all the array values are shown to store zero initially, because that is the default for int instance variables, and the members of arrays are much like these. After prompting the user for a new value to store in each index (I entered 3, 8, 12, -5, 7), the debugger shows

int

  int[] a = new int[5];

We can also construct an array by declaring all the values that it must contain; in such a case, the length of the array is automatically inferred from the number of values. So, for example, we can declare and initialize an array variable storing a reference to an object containing the five int values 4, 2, 0, 1, and 3 (in that order) by the declaration

  int[] a = new int[] {
     4,
     2,
     0,
     1,
     3
  };

a₂

  if (a[2] == 0)
     ...some statement

a₂

  if (a[2] == 0)
     ...some statement

Although indexes are always integers, the values stored at each index depends on the type used to declare/construct the array. In this example, each index stores a reference to a String object. Finally, using what we learned above, we could have declared and intialized this variable and its array object in just one declaration: by either

String[] s = new String[] {
   new String("ABC"), new String("LMN"), new String("XYZ")
};

String[] s = new String[]{new String("ABC"), new String("LMN"), new String("XYZ")};

String[] s = new String[] {
   "ABC",
   "LMN",
   "XYZ"
};

int[]

  int sum = 0;
  for (int i = 0; i < a.length; i++)
     sum += a[i];
  System.out.println("Sum = " + sum);

The following code prints on one line, separated by spaces, all the values stored in an array. After all the values are printed (and the loop terminates), it ends the line. Notice that the same for loop code is used (this time to examine and print, not add together) every member stored in the array object; only the body of the loop (telling Java what to do with each indexed member) is different.

  for (int i = 0; i < a.length; i++)
     System.out.print(a[i] + " ");
  System.out.println();

Below, a more interesting version of this loop prints a comma-separated list of values stored in the array: the last one is not followed by a comma, but by a new line. The String to catenate is decided by a conditional expression.

  for (int i = 0; i < a.length; i++)
     System.out.print(a[i] + (i < a.length - 1 ? "," : "\n"));

The following code computes and prints the maximum value stored in an array. Because the original maximum value comes from index 0 in the array (the first value stored in the array), the for loop starts at index 1 (the second value stored in the array).

  int max = a[0];
  for (int i=1; i<a.length; i++) if (a[i]>max)
     max = a[i];
     System.out.println("Max = " + max);

max

  int max = Integer.MIN_VALUE;
  for (int i=1; i<a.length; i++) if (a[i]>max)
     max = a[i];
     System.out.println("Max = " + max);

String

  String s = Prompt.forString("Enter Name");
  int asciiCharSum = 0;
  for (int i=0; i<s.length(); i++) asciiCharSum +=s.charAt(i); //Implicit conversion char->int
     System.out.println(s +"'s ASCII sum = " + asciiCharSum);

Suggestion : 5

In the above snippet of code, we have defined a class and declared some variables to the class. We have then instantiated the class and printed the required output for the users. We can observe that these class variables can contain any data type available to us in Python. As in the above program, we have strings and an integer.,Moreover, we can also observe that the object of myAnimal is accessible to all the variables in the class and print them out when we execute the program.,In the above snippet of code, we have defined a Student class and defined some variables as id, name, and age passed as arguments within the constructor method. We have then instantiated the class and print the values of the instance variables for the users.,In the above snippet of code, we have defined a class as "Animal" and declared the class variable. We have then instantiated the class with the my_Animal object and printed the final value for the users. As a result, the program returns the value of the class variable.

Name of the Animal: Lion

Name of the Animal: Lion
This Animal is found in: Jungle
This Animal is a: Carnivore
Population of this Animal: 20000 approx.

Roll Number of the Student: 102
Name of the Student: Sam
Age of the Student: 13

Suggestion : 6

The program uses the method getClass inherited from class Object, and the field length. The result of the comparison of the Class objects in the first println demonstrates that all arrays whose components are of type int are instances of the same array type, which is int[]. ,An array creation expression specifies the element type, the number of levels of nested arrays, and the length of the array for at least one of the levels of nesting. The array's length is available as a final instance variable length. ,All the components of an array have the same type, called the component type of the array. If the component type of an array is T, then the type of the array itself is written T[]. ,where the string "[I" is the run-time type signature for the class object "array with component type int".

1._

int[] ai; // array of int
short[][] as; // array of array of short
short s, // scalar short
aas[][]; // array of array of short
Object[] ao, // array of Object
   otherAo; // array of Object
Collection < ? > [] ca; // array of Collection of unknown type

The declarations above do not create array objects. The following are examples of declarations of array variables that do create array objects:

Exception ae[] = new Exception[3];
Object aao[][] = new Exception[2][3];
int[] factorial = {
   1,
   1,
   2,
   6,
   24,
   120,
   720,
   5040
};
char ac[] = {
   'n',
   'o',
   't',
   ' ',
   'a',
   ' ',
   'S',
   't',
   'r',
   'i',
   'n',
   'g'
};
String[] aas = {
   "array",
   "of",
   "String",
};

For example:

byte[] rowvector, colvector, matrix[];

This declaration is equivalent to:

byte rowvector[], colvector[], matrix[][];

For example, the local variable declaration:

int a, b[], c[][];

is equivalent to the series of declarations:

int a;
int[] b;
int[][] c;

Brackets are allowed in declarators as a nod to the tradition of C and C++. The general rules for variable declaration, however, permit brackets to appear on both the type and in declarators, so that the local variable declaration:

float[][] f[][], g[][][], h[]; // Yechh!

1._

class Gauss {
   public static void main(String[] args) {
      int[] ia = new int[101];
      for (int i = 0; i < ia.length; i++) ia[i] = i;
      int sum = 0;
      for (int e: ia) sum += e;
      System.out.println(sum);
   }
}

This program produces the output:

1._

class Point {
   int x, y;
}
class ColoredPoint extends Point {
   int color;
}
class Test {
   public static void main(String[] args) {
      ColoredPoint[] cpa = new ColoredPoint[10];
      Point[] pa = cpa;
      System.out.println(pa[1] == null);
      try {
         pa[0] = new Point();
      } catch (ArrayStoreException e) {
         System.out.println(e);
      }
   }
}

This program produces the output:

true
java.lang.ArrayStoreException: Point

1._

class Test {
   public static void main(String[] args) {
      int ia[][] = {
         {
            1,
            2
         },
         null
      };
      for (int[] ea: ia) {
         for (int e: ea) {
            System.out.println(e);
         }
      }
   }
}

This program produces the output:

1
2