Computer Science · Ch 5 — Getting Started with Python
Summary
Summary
- Python is an open-source, high-level, interpreter-based language usable for a multitude of scientific and non-scientific computing purposes.
- Python works in two execution modes: interactive mode executes one statement at a time at the
>>>prompt, while script mode saves multiple statements in a.pyfile that can be run and reused. - Keywords are reserved words with fixed meanings; Python is case-sensitive, so keywords and identifiers must be written exactly.
- An identifier is a user-defined name given to a variable or constant in a program. It must start with a letter or underscore, may contain letters, digits and underscores, and cannot be a keyword or contain special symbols.
- A variable refers to an object stored in memory; variable declaration is implicit — a variable is created the moment it is first assigned a value. Trying to use a variable that has not been assigned a value gives an error.
- Comments (starting with
#) are non-executable statements in a program, added to make the source code easier for humans to understand. - Every value in Python is an object with a unique identity (ID), retrievable with
id(). - There are several data types in Python — integer, boolean, float, complex, string, list, tuple, set, None and dictionary.
- Data types are classified as mutable (lists, sets, dictionaries — values can change after creation) or immutable (integers, floats, booleans, complex, strings, tuples — values cannot change; updating rebuilds the object).
- Operators are constructs that manipulate the values of operands. Operators may be unary (one operand) or binary (two operands). Python provides arithmetic, relational, assignment, logical, identity (
is,is not) and membership (in,not in) operators. - An expression is a combination of values, variables and operators, and it always evaluates to a value. Evaluation order follows operator precedence; parentheses override precedence, and operators of equal precedence apply left to right.
- A statement is a unit of code that the Python interpreter can execute, such as an assignment or a print statement.
- Python has the
input()function for taking user input — it always returns a string, which must be typecast (e.g. withint()) for arithmetic. - Python has the
print()function to output data to a standard output device; optionalsepandendparameters control the separator between values and the string appended after the last value. …