Python Syntax Rules

I introduced Python’s syntax model in Chapter 10. Now that we’re stepping up to larger statements like the if, this section reviews and expands on the syntax ideas introduced earlier. In general, Python has a simple, statement-based syntax. However, there are a few properties you need to know about:

 

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  • Statements execute one after another, until you say otherwise. Python normally runs statements in a file or nested block in order from first to last, but statements like if (and, as you’ll see, loops) cause the interpreter to jump around in your code. Because Python’s path through a program is called the control flow, statements such as if that affect it are often called control-flow statements.
  • Block and statement boundaries are detected automatically. As we’ve seen, there are no braces or “begin/end” delimiters around blocks of code in Python; instead, Python uses the indentation of statements under a header to group the statements in a nested block. Similarly, Python statements are not normally terminated with semicolons; rather, the end of a line usually marks the end of the statement coded on that line.
  • Compound statements = header + “:” + indented statements. All compound statements in Python follow the same pattern: a header line terminated with a colon, followed by one or more nested statements, usually indented under the header. The indented statements are called a block (or sometimes, a suite). In the if statement, the elif and else clauses are part of the if, but they are also header lines with nested blocks of their own.
  • Blank lines, spaces, and comments are usually ignored. Blank lines are ignored in files (but not at the interactive prompt, when they terminate compound statements). Spaces inside statements and expressions are almost always ignored (except in string literals, and when used for indentation). Comments are always ignored: they start with a # character (not inside a string literal) and extend to the end of the current line.
  • Docstrings are ignored but are saved and displayed by tools. Python supports an additional comment form called documentation strings (docstrings for short), which, unlike # comments, are retained at runtime for inspection. Docstrings are simply strings that show up at the top of program files and some statements. Python ignores their contents, but they are automatically attached to objects at runtime and may be displayed with documentation tools. Docstrings are part of Python’s larger documentation strategy and are covered in the last chapter in this part of the book.

As you’ve seen, there are no variable type declarations in Python; this fact alone makes for a much simpler language syntax than what you may be used to. However, for most new users the lack of the braces and semicolons used to mark blocks and statements in many other languages seems to be the most novel syntactic feature of Python, so let’s explore what this means in more detail.