{"title":"First Python Program","description":"","content":"The fastest way to get comfortable with Python is to write a tiny program, run it, and see what happens. This lesson walks through that loop using a small e-commerce program we'll build on later in the course. You'll see two different ways to run Python code, write your first script, and learn how to read the error messages you'll inevitably hit.\n\n---\n\n# Hello, World\n\nOpen a plain text editor or your IDE and create a file named `hello.py`. Type this single line:\n\n\n```python\nprint(\"Hello, World!\")\n```\n\n\nSave it. Open a terminal, change into the directory that holds `hello.py`, and run:\n\n\n```shell\npython hello.py\n```\n\n\nThat's a complete Python program. One line. No class, no `main` method, no semicolons, no boilerplate. The `print()` function writes its argument to the terminal and adds a newline at the end.\n\nA few things to notice before we go further:\n\n- Python is case-sensitive. `print` is a built-in, `Print` is not.\n- Statements don't end with semicolons. A newline ends a statement.\n- There are no curly braces. Python uses indentation to group code, which we'll see later in the course.\n\nThe pipeline from your source file to terminal output looks like this:\n\n\n```mermaid\nflowchart LR\n A[hello.py
source file]:::cyan --> B[python
interpreter]:::orange\n B --> C[Reads and runs
each line]:::teal\n C --> D[Terminal output]:::green\n\n classDef cyan fill:#00ceff,stroke:#000,color:#000\n classDef orange fill:#ffa94d,stroke:#000,color:#000\n classDef teal fill:#38d9a9,stroke:#000,color:#000\n classDef green fill:#69db7c,stroke:#000,color:#000\n```\n\n\nYou give `python` a file, the interpreter reads each line in order, and anything you `print` shows up in your terminal. There's no separate compile step, no `.class` file, no extra command. We'll open up what's actually happening inside the interpreter in the next lesson.\n\n---\n\n# The Interactive REPL\n\nYou don't always need a file. Python ships with an interactive shell called the REPL, which stands for **Read, Eval, Print, Loop**. Type an expression, press Enter, and Python evaluates it and shows the result. It's the fastest way to try out small snippets.\n\nStart it by running `python` with no arguments:\n\n\n```shell\npython\n```\n\n\nYou'll see something like this:\n\n\n```shell\nPython 3.12.1 (main, Dec 11 2023, 16:30:14) [Clang 15.0.0] on darwin\nType \"help\", \"copyright\", \"credits\" or \"license\" for more information.\n>>>\n```\n\n\nThe `>>>` is the REPL prompt. It means Python is waiting for you to type something. Try a few things:\n\n\n```python\n>>> print(\"Hello, World!\")\nHello, World!\n>>> 2 + 3\n5\n>>> \"Wireless Mouse\"\n'Wireless Mouse'\n>>> price = 19.99\n>>> price * 3\n59.97\n```\n\n\nNotice two things here. First, you can `print()` exactly like you would in a script. Second, when you type an expression by itself (like `2 + 3` or `\"Wireless Mouse\"`), the REPL evaluates it and shows the result, no `print()` needed. That's the \"Print\" part of REPL: anything you type that produces a value gets echoed back.\n\nThis shortcut only works in the REPL. Inside a `.py` script, `2 + 3` on its own line does nothing visible. You'd have to write `print(2 + 3)` to see the result.\n\nTo leave the REPL, type `exit()` and press Enter, or press **Ctrl+D** (on macOS and Linux) or **Ctrl+Z** then Enter (on Windows).\n\n\n```python\n>>> exit()\n```\n\n\nThe REPL is great for quick experiments: testing what a function returns, checking the type of something, or trying a one-liner before pasting it into a script.\n\n---\n\n# Running a Script File\n\nThe REPL is fine for one-offs, but real programs live in files. A Python script is just a text file with a `.py` extension. The naming convention is lowercase with underscores, like `hello.py` or `order_summary.py`.\n\nHere's the workflow:\n\n1. Create a file ending in `.py`.\n2. Write Python code in it.\n3. Run it with `python filename.py`.\n\nLet's say `hello.py` contains:\n\n\n```python\nprint(\"Hello, World!\")\nprint(\"Welcome to MyShop\")\nprint(\"Free shipping on orders over $50\")\n```\n\n\nRunning it:\n\n\n```shell\npython hello.py\n```\n\n\nA few practical points:\n\n- You don't need to be in the script's directory, but it's the easiest setup. If you're elsewhere, give the full path: `python /Users/you/projects/hello.py`.\n- On some systems the command is `python3` instead of `python`. If `python` doesn't work or points to an older version, use `python3`.\n- The interpreter reads the file top to bottom and executes each statement in order. There's no \"main function\" that Python looks for. The first line runs first, then the second, and so on.\n\nThis is the main difference from many other languages. There's no required entry-point function, no required class, no boilerplate. A Python file with one line is a complete program.\n\n\n| Mode | How you start it | When to use |\n| --- | --- | --- |\n| REPL | `python` | Quick experiments, exploring a library, testing a one-liner |\n| Script | `python file.py` | Anything you want to save, share, or run more than once |\n\n\n\n> **INFO**\n>\n> **Cost:** Starting the Python interpreter takes a moment (typically 30-100ms). For tiny snippets you'd run hundreds of times, prefer the REPL or a long-running process. For everything else, a script is fine.\n\n\n---\n\n# Your Second Program\n\nOne line of output is fine, but real programs do something with data. Here's a slightly bigger example: a tiny order summary that prints the contents of a customer's cart.\n\nCreate a file named `order_summary.py`:\n\n\n```python\nproduct_name = \"Wireless Mouse\"\nquantity = 2\nprice = 19.99\ntotal = quantity * price\n\nprint(\"Order Summary\")\nprint(product_name, \"x\", quantity)\nprint(\"Total:\", total)\n```\n\n\nRun it:\n\n\n```shell\npython order_summary.py\n```\n\n\nWalking through it line by line:\n\n- The first three lines store values in names: `product_name`, `quantity`, and `price`. The next lesson section covers variables in depth, so for now think of `=` as \"give this name the value on the right\".\n- `total = quantity * price` multiplies the quantity by the price and stores the result in `total`. The arithmetic happens before the assignment.\n- `print(\"Order Summary\")` writes the header line.\n- `print(product_name, \"x\", quantity)` is the interesting one. `print()` can take multiple arguments, separated by commas. It prints them in order with a single space between each. So `Wireless Mouse`, then a space, then `x`, then a space, then `2`.\n- `print(\"Total:\", total)` works the same way. It prints `Total:`, a space, then the number stored in `total`.\n\nThat default space between arguments is the value of `print()`'s `sep` parameter, which is `\" \"` (a single space) unless you override it. You can pass `sep=\"|\"` or `sep=\"\"` if you want something else.\n\nEach `print()` call also adds a newline at the end. That's why each piece of output appears on its own line. That newline is controlled by `print()`'s `end` parameter, which defaults to `\"\\n\"`. If you pass `end=\"\"`, the next `print()` continues on the same line.\n\nYou don't need semicolons between statements. The end of a line is the end of a statement. You also don't need to declare the types of `product_name`, `quantity`, or `price`. Python figures out that `product_name` is text, `quantity` is a whole number, and `price` is a decimal number from the values you assign.\n\n---\n\n# Reading Error Messages\n\nYou will make typos. Everyone does. The good news is that Python's error messages are usually short and point right at the problem. Learning to read them saves a lot of time. Here are three you'll meet in your first week.\n\n### `NameError` from a typo\n\n**What's wrong with this code?**\n\n\n```python\nPrint(\"Hello, World!\")\n```\n\n\nPython prints:\n\n\n```shell\nTraceback (most recent call last):\n File \"/Users/you/hello.py\", line 1, in \n Print(\"Hello, World!\")\n ^^^^^\nNameError: name 'Print' is not defined\n```\n\n\nThe traceback tells you exactly what happened. Line 1 of `hello.py` uses the name `Print`, but Python doesn't know what `Print` is. The built-in function is `print` with a lowercase `p`. Python is case-sensitive, so `Print`, `PRINT`, and `print` are three different names.\n\n**Fix:** Use lowercase `print`:\n\n\n```python\nprint(\"Hello, World!\")\n```\n\n\n`NameError: name 'X' is not defined` almost always means a typo in a name or a variable you haven't created yet. Read the line, find the name Python is pointing at, and check the spelling and capitalization.\n\n### `SyntaxError` from missing parentheses\n\n**What's wrong with this code?**\n\n\n```python\nprint \"Hello, World!\"\n```\n\n\nThis is what Python 2 used to allow, but Python 3 requires parentheses around the argument. Python reports:\n\n\n```shell\nFile \"/Users/you/hello.py\", line 1\n print \"Hello, World!\"\n ^^^^^^^^^^^^^^^^^^^^^\nSyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?\n```\n\n\nThe error happens before any code runs. Python tries to parse the file and gives up because the syntax is wrong. The `Did you mean print(...)?` hint is the interpreter trying to help.\n\n**Fix:** Wrap the argument in parentheses:\n\n\n```python\nprint(\"Hello, World!\")\n```\n\n\nA `SyntaxError` always means the file doesn't follow Python's grammar. None of your code runs until the whole file parses successfully.\n\n### `IndentationError` from a stray space\n\nPython uses indentation to group code, so leading whitespace is significant. A line that's indented when it shouldn't be triggers an error.\n\n**What's wrong with this code?**\n\n\n```python\nprint(\"Order Summary\")\n print(\"Wireless Mouse\")\n```\n\n\nThe second line starts with four spaces for no reason. Python reports:\n\n\n```shell\nFile \"/Users/you/hello.py\", line 2\n print(\"Wireless Mouse\")\nIndentationError: unexpected indent\n```\n\n\n**Fix:** Remove the leading spaces so both lines start at the same column:\n\n\n```python\nprint(\"Order Summary\")\nprint(\"Wireless Mouse\")\n```\n\n\nYou'll meet `IndentationError` more often once you start writing functions, loops, and `if` statements, since those rely on indentation to mark their bodies. For now, the rule is simple: top-level lines start at column zero, with no leading spaces or tabs.\n\n---\n\n# A Quick Note on Comments\n\nYou'll see lines starting with `#` in many Python examples. Anything from `#` to the end of the line is a comment. Python ignores it.\n\n\n```python\n# This script prints an order summary.\nproduct_name = \"Wireless Mouse\"\nprice = 19.99\nprint(product_name, price) # Inline comments work too.\n```\n\n\nUse comments sparingly, mostly to explain why something is happening, not what. The next section covers comments and docstrings in more detail.\n\nOne more piece of trivia worth knowing: on Unix-like systems (macOS, Linux), the very first line of a script can be a **shebang** like `#!/usr/bin/env python3`. It tells the operating system which interpreter to use when the script is run directly as an executable (e.g., `./hello.py`). It's optional, doesn't affect `python hello.py`, and isn't needed on Windows. You'll see it in older codebases and command-line tools, but you can ignore it for the rest of this course.\n\n---\n\n# Summary\n\n- A Python program can be as short as one line. There's no required class, no entry-point function, no semicolons.\n- You can run Python code two ways: interactively in the REPL (`python` with no arguments), or as a script (`python file.py`).\n- The REPL echoes the value of any expression you type. Scripts don't, you have to call `print()` to see output.\n- `print()` takes one or more arguments and separates them with a space by default. It also adds a newline at the end. Both behaviors are controlled by the `sep` and `end` parameters.\n- Python is case-sensitive: `print` is not the same as `Print`. Typos in names raise `NameError`.\n- Read error messages from the bottom up. The last line tells you the type and message, and the traceback above it points to the exact line where the problem happened.\n- Comments start with `#` and run to the end of the line. Use them sparingly to explain why, not what.\n\nYou can now write, save, and run Python code. What you probably don't know yet is what `python hello.py` actually does between you pressing Enter and the output appearing. In the next lesson, _How Python Works (Interpreter, Bytecode, PVM)_, we look at the interpreter pipeline, the bytecode it produces, and the Python Virtual Machine that executes it.","pageType":"python"}

First Python Program

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