Identifiers and Kewyords

Identifiers are names given by programmers to variables, functions, attributes, and other objects.

Identifier Naming Rules

1. Case-sensitive: acd, ACD, and acD are all different identifiers.
2. Start with a letter or underscore: Must begin with a letter (a-z, A-Z) or underscore _, but not a digit. For example, acd and _acd are valid, while 3acd is not.
3. Subsequent characters: Can be letters, digits, or underscores.
4. Avoid reserved words: Keywords and built-in function names cannot be used as identifiers.
5. Special names: Identifiers starting and ending with double underscores have special meaning in Python; avoid using them unless necessary.

Common Naming Conventions in Practice

Element	Convention
Module, Package	All lowercase with underscores between words
Function	All lowercase with underscores between words
Class	CapWords (CamelCase), e.g., MyClassTest
Constant	All uppercase with underscores between words

Discovering Keywords

Method 1: In PyCharm, run:

help()

Then type keywords in the help> prompt.

Method 2: Execute the following code:

import keyword
print(keyword.kwlist)

Output:

['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']

Comments

Comments are text ignored by the Python interpreter, used for code explanation.

Single-line Comment

Use # before the comment text. In PyCharm, shortcut: Ctrl + /.

Multi-line Comment

Enclose text between three single quotes ''' or three double quotes """. This is technically a string literal, but if not assigned to a variable, it acts as a comment and is eventually garbage-collected.

Varialbes and Constants

Variables

Python does not require explicit type declaration. A variable is created when a value is assigned to an identifier. The variable's type is determined by the assigned value.

y = 20
y = True  # y now holds a boolean value

Deleting Variables

Use the del statement to remove a variable. After deletion, accessing it raises a NameError.

x = 10
del x
print(x)  # NameError: name 'x' is not defined

A variable must be declared before use. If an object has no references, it becomes eligible for garbage collection.

Constants

Python lacks a syntactic way to define true constants. By convention, variables are treated as constants and should not be modified after assignment.

Statements

A single line of code forms a statement. Semicolons are optional.

Chained Assignment

a = b = c = 10  # 10 assigned to a, b, and c

Series Unpacking Assignment

a, b, c = 10, 20, 30
print(a, b, c)  # Outputs: 10 20 30

Variable Swapping

Python allows swapping values without a temporary variable:

x, y = 100, 300
x, y = y, x
print(x, y)  # Outputs: 300 100

Line Continuation with Backslash

Long lines can be split using \:

text = "abcdefghijkl\
mnopqrstuv\
wxyzaaaaa"
print(text)

Output:

abcdefghijklmnopqrstuvwxyzaaaaa

Indentation

Python uses indentation to define code blocks, similar to braces {} in C-like languages. Consistent indentation is required.

• Any number of spaces is allowed, but must be consistent within a block.
• In PyCharm, pressing Tab inserts 4 spaces.
• Do not mix tabs and spaces.

Modules

A module is a .py file containing Python code. It is the smallest unit of code organization. A Python program executes statements in a module sequentially.

Importing a Module

Approach 1: Import entire module

import module_name
# usage: module_name.function()

Approach 2: Import specific elements

from module_name import function_name
# usage: function_name()

Packages

Packages help organize modules and avoid name conflicts.

Creating a Package

In the project, right-click → New → Python Package, then enter a name. A package always contains an __init__.py file (can be empty) to signal to the interpreter that the directory is a package.

Importing a Package

import com.hello as a
# usage: a.function_name()

This imports all code elements from the com.hello module, aliased as a.