Python Syntax and Statements

An expression is any piece of code that produces a value: 14, packets + incoming, name.upper() and available >= 20 are all expressions. A statement is a complete instruction that does something: it stores a value, makes a decision, repeats work or defines a function.

packets + 6             # expression: produces 20, then the value is thrown away
available = packets + 6 # assignment statement: stores the value under a name
print(available)        # expression statement: calls a function for its effect

The distinction explains several error messages. You can put an expression anywhere a value is expected, such as inside print() or after return. A plain assignment with = is a statement, not an expression, so if count = 5: is a SyntaxError. Python 3.8 added the walrus operator :=, which assigns and produces a value in one step, if (n := len(items)) > 10:, but beginners rarely need it.

In the interactive shell, typing an expression shows its value straight away. In a script, an expression on its own line is calculated and discarded, which is why scripts use print() to display results.

Statements such as if, for, while, def and class end with a colon and are followed by a block: one or more lines indented further than the header. The block ends at the first line that returns to the header's level.

if available >= 20:
    print("Ready for the seed swap")
    print("Open the sign-up sheet")
print("Inventory checked")

The first two print() calls run only when the condition is true; the last one always runs. Other languages mark blocks with braces, but in Python the indentation is the structure, so it isn't optional decoration.

PEP 8, Python's style guide, recommends four spaces per level. The one hard rule is consistency: every line in a block must use the same indentation, and mixing tabs and spaces in a way Python can't resolve raises TabError. Configure your editor to insert four spaces when you press Tab and the problem disappears.

Python normally ends a statement at the end of a line. When a calculation, list or function call is too long to read comfortably, wrap it in brackets: inside (), [] or {}, line breaks are ignored until the closing bracket.

available = (
    packets_on_shelf
    + packets_donated
    - packets_reserved
)
crops = [
    "tomato", "bean",
    "squash", "chard",
]

This is called implicit line joining and is the preferred way to split long lines. A backslash at the end of a line also continues it, but a single invisible space after the backslash breaks it, so most style guides avoid it.

Several short statements can share one line if you separate them with semicolons, a = 1; b = 2, but this hides structure and is discouraged. One statement per line keeps code easy to scan and makes tracebacks point at the exact step that failed.

Before running anything, Python parses the whole file: it checks that the text follows the grammar and turns it into an internal structure. If the grammar is broken anywhere, nothing runs at all, not even the lines above the mistake. That is the key difference between a SyntaxError and a run-time error such as NameError, which only appears when the faulty line is reached.

  File "inventory.py", line 3
    if available >= 20
                      ^
SyntaxError: expected ':'

The caret points to where Python realised something was wrong, which is not always where the mistake is. An unclosed bracket or quote on line 2 is often reported on line 3 or later, because Python kept reading in the hope that the bracket would close. Python 3.10 and later give much clearer messages, such as '(' was never closed, so keep your interpreter up to date. When the highlighted line looks fine, check the line above it.

Task: build a planting inventory with a multiline expression and two actions inside a condition. Store packets on the shelf, donated packets and reserved packets in three variables, calculate the number available using a bracketed expression over several lines, and, if at least 20 are available, print two messages from inside the if block. Finish with a line that always prints.

One possible solution:

on_shelf = 15
donated = 9
reserved = 4

available = (
    on_shelf
    + donated
    - reserved
)

if available >= 20:
    print(f"{available} packets available")
    print("Announce the seed swap")
print("Inventory complete")

With these numbers, available is 20, so all three lines print. Change reserved to 6 and only the last line prints. The worked check for this lesson asks what defines a block in Python: the answer is consistent indentation after a line ending in a colon. Indent the second print() by two spaces instead of four and Python raises IndentationError: unindent does not match any outer indentation level.

The Python language reference on lexical analysis defines exactly how lines, indentation and brackets are read, and PEP 8 gives the community's layout conventions. Continue with comments to document the code you write, then practise with the Python exercises and Python quiz. The study roadmap shows where this lesson fits in the full course.