How Python reads your code

Imagine handing a checklist to a new colleague who follows every instruction exactly as written, in order, and never guesses what you meant. That is how Python treats your program. It is very fast and completely literal.

Knowing exactly how Python reads a file saves you a lot of confusion later. In this lesson you'll see what a single instruction looks like, the order instructions run in, how to leave notes for human readers, and the few small details (capital letters, spaces at the start of a line) that Python is strict about.

One statement at a time

Each complete instruction in a program is called a statement. In the programs you'll write in this track, a statement is usually one line. print("Hello") is a statement.

When you press Run, Python starts at the first line, carries out that statement, then moves to the next one. It never skips ahead and never goes back on its own. If a statement fails, Python stops right there and reports an error, and the lines after it do not run.

The first two lines work, so their output appears. On line 3, Python finds the name prnt, which it does not know, and stops with a NameError. Line 4 never runs. You'll learn to read error messages properly in Reading error messages; for now, notice that the error tells you which line caused the problem.

Comments: notes for humans

A comment starts with #. Python ignores everything from the # to the end of that line. Comments are for people: they explain what the code is for, so that you (or a colleague) can understand it next week.

A comment can sit on its own line or at the end of a line of code.

Line 3 is a print statement with a # in front, so Python treats the whole line as a comment. Putting # in front of a line to switch it off without deleting it is called commenting out. It is handy when you want to try the program without a line but may want it back later.

A # inside quotes is ordinary text, not a comment: print("Room #4") prints Room #4.

Blank lines in code versus blank lines in output

Python ignores blank lines in your code. Use them to group related lines, the way paragraphs group sentences. They do not produce empty lines in the output.

To print an empty line, call print() with nothing between the parentheses.

Capital letters matter

Python is case sensitive: it treats uppercase and lowercase letters as different. print, Print and PRINT are three different names, and only print is built into Python. Writing Print("Hi") stops the program with a NameError, exactly like the spelling mistake above.

Inside quotes, capitals are kept exactly as you type them: print("ABC") and print("abc") print different text.

Spaces at the start of a line

In many languages, spaces at the start of a line are only decoration. In Python they are part of the meaning. The spaces at the start of a line are called indentation, and Python uses them to group lines that belong together. You'll use that properly when you learn about conditions and loops.

For now the rule is short: every line in your programs should start at the left edge, with no spaces before it. An unexpected indent stops the program before it even starts.

Notice that this time not even the first line printed. Before running anything, Python reads the whole file to check that it follows the rules of the language (its syntax). An indentation problem breaks those rules, so Python refuses to start. A misspelled name like prnt is different: the file is valid Python, and the problem only shows up when that line runs.