Python: Core Data Types and Operations

Core Data Types

• The Class datatype is not a core datatype
• When a function returns nothing, the Python shell throws a NoneType object
• The output of the following commands will be "he"

>>>str="hello"
>>>str[:2]
>>>

• The following code will run without errors:

round(45.8)
round(6352.898,2)

• A function id returns a unique integer
• The following code operations ensure that x has an integer value in Python 3.xx:

x = 13 // 2
x = int(13/2)

• NameError is the error that occurs when you execute the following code:

apple = mango

• The L = [1, 23, ‘hello’, 1] object is a List datatype
• A Dictionary dataype is used to store values in terms of key and value
• The following code results in a SyntaxError: "He said, "Yes!"" '3\'
• The code will print "tom\ndick\nharry" under these conditions:

print("" tom
\ndick
\nharry"" )
print(‘tom\ndick\nharry’)

• The average value of the following code is 85.0, as a decimal value to appear is the output:

>>>grade1 = 80
>>>grade2 = 90
>>>average = (grade1 + grade2) / 2

• The following code will print hello-how-are-you:

print(‘hello-‘ + ‘how-are-you’)
print(‘hello’ + ‘-‘ + ‘how’ + ‘-‘ + ‘are’ + ‘-‘ + ‘you’)

• The return value of trunc() is an integer

Basic Operators

• The correct operator for power(x^y) is X**y, i.e. 2**3=8
• // performs floor division
• The order of precedence in Python is Parentheses, Exponentiation, Division, Multiplication, Addition, Subtraction (PEDMAS)
• The answer of 22 % 3 is 1 since modulus operator gives remainder
• Mathematical operations cannot be performed on strings, even if they look like integers
• Operators with the same precedence are evaluated from left to right
• The output of the expression 3*1**3 is 3, because exponentiation has higher precedence than multiplication, the solution is 1**3 = 1; 3*1 = 3
• Addition and Subtraction or Multiplication and Division have the same precedence
• Int(x) converts the variable x to an integer
• Parentheses have the highest precedence in an expression

Variable Names

• Python is case-sensitive when dealing with identifiers

• Identifiers can be of any length

• Variable names should not begin with a number

• Local variable names beginning with an underscore are discouraged to indicate variables that must not be accessed from outside the class

• eval is not a keyword, and can be used as a variable

• True, False, and None are capitalized, while all other keywords in Python are in lowercase

• Variable names in Python can have unlimited length

• Spaces are not allowed in variable names

• The word in is a keyword, and cannot be used as a variable

Regular Expressions

• The re module in Python is for regular expressions, import with import re
• re.compile(str) creates a pattern object, and converts a given string into a pattern object
• The function re.match matches a pattern at the start of the string, and will look for the pattern at the beginning returning None if its not found
• The function re.search matches a pattern at any position in the string, will look for the pattern at any position in the string
• The function matched.groups() returns all the subgroups that have been matched
• The function matched.group() returns the entire match
• The function matched.group(2) returns the particular subgroup
• These regex functions return all the matches in a dictionary:

sentence = 'horses are fast'
regex = re.compile('(?P<animal>\w+) (?P<verb>\w+) (?P<adjective>\w+)')
matched = re.search(regex, sentence)
print(matched.groupdict())

• These regex functions return all the subgroups that have been matched:

sentence = 'horses are fast'
regex = re.compile('(?P<animal>\w+) (?P<verb>\w+) (?P<adjective>\w+)')
matched = re.search(regex, sentence)
print(matched.groups())

• This function returns the particular subgroup for regex:

sentence = 'horses are fast'
regex = re.compile('(?P<animal>\w+) (?P<verb>\w+) (?P<adjective>\w+)')
matched = re.search(regex, sentence)
print(matched.group(2))

Numeric Types

• Print 0.1 + 0.2 == 0.3 gives the output False
• k = 2 + 3l is not a complex number
• The type of inf is float
• ~4 evaluates to -5

While and For Loops

• The function upper() does not perform in place modification to a string.

• A code inside a while True code block will execute until a break occurs.

• The else part in a while loop is not executed if the loop breaks

• A NameError will occur if the variables are not defined

• The value of 'i' or 'x' is not altered if a condition always evaluates to True

• The range() is computed only at the time of entering the loop

• An object of type int is not iterable.

• These functions use the keys of the dictionary and prints the values:

d = {0: 'a', 1: 'b', 2: 'c'}
for x in d.keys():
print(d[x])

• 0O11 is an octal number.

• A TypeError arises when using the center() method incorrectly: The fill character must be exactly one character long.

• The output with a string.translate() function is the output with modified indexes

• Variables values aren't automatically retained by default on new iterations:

def f(i, values = []):
      values.append(i)
      return values

f(1)
f(2)
v = f(3)
print(v)

String Functions

• The + operator concatenates strings and, unlike the * symbol, cannot be used with numbers
• The slice() function is performed on a string
• string.ascii_lowercase+string.ascii_upercase is the same as string.ascii_letters
• -1 corresponds to the last index
• \ is an escape sequence
• String literals that are separated by white space are joined together
• The character values for 0xA and 0xB and 0xC are hexadecimal
• Code involving id attributes can show whether strings are in fact the same string or just have the same value

Numerical Operations

• Numerical comparison > operators and arithmetic operators such as + cannot be used with dictionaries

List Methods

• The following list methods are available on lists:

print(list1[0]) # prints first item in list
print(list1[:2]) # prints the first two items in the list
print(list1[:-2]) # prints the list with the exceptions of the last two items
print(list1[4:6]) # prints items at index 4 to 6

• Slicing is allowed in lists just as in the case of character strings
• Max returns the maximum element in the list
• Min returns the minimum element in the list
• list1.reverse() will print the list is reverse
• listExample.extend([34, 5]) will extend at the new list
• listExample.pop(1) will remove the element at the position as specified in the parameter
• With the string += [1] statement will append all the elements individually into a new list
• A list will passed in append:
  • Numbers = [1, 2, 3, 4]
  • numbers.append([5,6,7,8]) len(number) = 5 because 1 list is appending so length is 5.
• + will append the element to the list
• Tuples can be used for keys into dictionary because tuples can have mixed length and the other the items in the tuple is considered when comparing the equality of the keys
• SyntaxError, not in isn’t allowed in loop

Tuples

• Tuples are characterised by their round brackets, ()
• Values cannot be modified in the case of a tuple
• Slicing in tuples takes place just as in character strings, to get a t = (1,2,4,3) you can use t[1:3] = (2,4)
• The * operator concatenates tuples