More on Variables, Operators and Expressions

Okay, let’s delve deeper into variables, operators, and expressions in Python, drawing from the sources and our previous conversation.

Building on our discussion of variables as names that refer to values or objects, these variables are frequently used within expressions and are manipulated by operators.

Expressions In Python, an expression is a combination of values, variables, and operators that the Python interpreter evaluates to produce a result. A simple value or a variable by itself can also be considered an expression. When a variable appears in an expression, it is immediately replaced with the object it currently refers to [Variable’s Perspective turn]. Examples of legal expressions include a variable name, a literal value, or combinations using operators. For instance, if n is a variable holding the value 17, both n and n + 25 are expressions.

Operators Operators are constructs that can manipulate the value of operands. Operands are the values on which an operator operates, such as the numbers 4 and 5 in the expression 4 + 5. Python supports various types of operators. The sources categorise them as follows:

• Arithmetic Operators: These perform mathematical operations.

  • + (Addition): Adds values on either side. Also used for string concatenation.
  • - (Subtraction): Subtracts the right operand from the left.
  • * (Multiplication): Multiplies values. Also used for string repetition.
  • / (Division): Divides the left operand by the right. This operator typically returns a float in Python 3.
  • // (Floor Division): Divides and rounds down to the nearest whole number (integer quotient).
  • % (Modulus): Returns the remainder after division.
  • ** (Exponentiation): Raises the left operand to the power of the right.

• Comparison (Relational) Operators: These compare values and return a Boolean result (True or False). You may have encountered these in previous computational thinking studies.

  • == (Equal to)
  • != (Not equal to). Note that the <> operator for inequality was removed in Python 3.0 as it was redundant.
  • > (Greater than)
  • < (Less than)
  • >= (Greater than or equal to)
  • <= (Less than or equal to)

• Logical Operators: These combine Boolean expressions.

  • and: If both operands are true, the condition becomes true.
  • or: If any of the two operands are non-zero (true), the condition becomes true.
  • not: Used to reverse the logical state of its operand. Note that and and or operators return an object, which is either the left or right operand, rather than always returning a simple True or False.

• Assignment Operators: The most basic is =, which assigns the value of the right operand to the left operand (a variable). Augmented assignment operators provide a shorthand for performing an operation and assignment simultaneously.

  • = (Simple assignment)
  • += (Add AND)
  • -= (Subtract AND)
  • *= (Multiply AND)
  • /= (Divide AND)
  • %= (Modulus AND)
  • **= (Exponent AND)
  • //= (Floor Division AND)

• Membership Operators: These test for membership in sequences (like strings, lists, tuples) or sets.

  • in: Evaluates to true if it finds a variable (or value) in the specified sequence or set.
  • not in: Evaluates to true if it does not find a variable (or value) in the specified sequence or set.

• Identity Operators: These compare the memory locations of two objects to see if they are the same object.

  • is: Evaluates to true if the operands refer to the same object.
  • is not: Evaluates to true if the operands do not refer to the same object. It’s important to distinguish is (object identity) from == (value equality).

• Bitwise Operators: These operate on individual bits. The source briefly mentions that the ^ operator is used for bitwise XOR in Python, not exponentiation as in some other languages.

Other operators mentioned include:

• Indexing ([]) and Slicing ([::]) operators for accessing elements or subsequences in sequences (strings, lists, tuples) and mappings (dictionaries). Slicing is considered a sequence operation.
• The Call operator (()) used to call functions, methods, or instances of classes that are callable.
• The Attribute reference operator (.) used to access attributes or methods of an object.
• The Ternary Expression (x if y else z) which provides a concise way to choose between two values based on a condition.
• lambda args: expression which creates anonymous functions and is considered an expression.
• yield x used in generator functions, which can sometimes be coded as an expression statement.
• The string formatting operator (%) and the .format() string method.

Operator Precedence In expressions with multiple operators, the order in which operations are performed is determined by operator precedence. Operators with higher precedence are evaluated before those with lower precedence. Parentheses () can be used to override the default precedence rules, forcing the expression within the parentheses to be evaluated first. The sources provide a table listing operators and their precedence.

Operator Overloading A powerful feature in Python is operator overloading, which allows objects created with classes to intercept and respond to operations that work on built-in types, such as addition, slicing, or printing. This is achieved by defining special methods (e.g., __add__ for +, __getitem__ for [], __str__ for str()) within a class. Operator overloading allows user-defined objects to support the same expected object interface as built-in types, making them compatible with functions or operations designed for those built-in types. The sources detail many of these special methods that implement operator behaviour.

Expression Statements While expressions compute a value, they can also be used as statements, known as expression statements. For example, simply typing a variable name or an expression in the interactive interpreter will cause Python to evaluate and echo its result; this serves as a shorthand for using print statements. Function calls and method calls are also common forms of expression statements. In Python 3.0, print itself is a function call, usually coded as an expression statement on a line by itself.

In essence, expressions combine variables (which refer to objects/values) and operators (which perform manipulations) to compute new values. Understanding the different types of operators, their precedence, and how they interact with various object types is fundamental to writing effective Python code.