Your First Python Program

Printing text and numbers

print() is the simplest way to see what your program is doing. Text must be inside quotes. You can use double quotes ("...") or single quotes ('...'), as long as the opening and closing quotes match. Numbers and calculations do not need quotes.

Each call to print() writes one line of output and then moves to a new line, so a program with three print() statements produces three lines.

print("Sample report")print('Temperature in kelvin:')print(25 + 273.15)

Several values on one line

You can give print() several values separated by commas. It prints them on the same line with a single space between each one. This is a convenient way to label a number.

print("Atoms in an FCC unit cell:", 4)

If you want something other than a space between the values, add sep= (short for separator) as the last item inside the brackets. An empty print() prints a blank line, which is useful for spacing out output.

print("Fe", "Cr", "Ni", sep=", ")print()print("Fe", "Cr", "Ni", sep="-")

Syntax

Every language has rules about how statements are written. In programming these rules are called syntax. Python's syntax is fairly relaxed, but a few rules matter from the start:

• Each statement normally goes on its own line.
• Brackets and quotes must come in matching pairs.
• Names are case-sensitive: print works, but Print and PRINT do not.
• Spaces at the start of a line (indentation) have a meaning in Python, so don't indent lines unless the code requires it. You will see where indentation is needed in the Control Flow module.

Comments

A comment starts with #. Python ignores everything from the # to the end of that line. Comments are notes for people reading the code, including you in six months' time when you have forgotten why you wrote it.

A comment can sit on its own line, usually just above the code it describes, or at the end of a line of code. Good comments explain why something is done or what units a value is in; they don't need to repeat what the code obviously does.

# Density of aluminium at room temperatureprint("Material: aluminium")print("Density (g/cm³):", 2.70)  # handbook value

Reading error messages

Everyone makes mistakes when programming, including experienced developers. When Python can't run your code it stops and shows an error message. Learning to read these messages is one of the most useful skills you can develop. Run this example to see one:

print("Hello, lab)

An error message has three useful parts:

• Where: the line number where Python noticed the problem.
• What kind: the error type, such as SyntaxError, NameError or TypeError.
• Why: a short description, such as unterminated string literal (a string that never ends).

Here the closing quote is missing, so Python thinks the text never ends. Add " before the closing bracket and run it again.

A NameError means Python doesn't recognise a name. It often comes from a typo or the wrong capitalisation:

Print("Hello, lab")