Basic Syntax

Input and output

Programming is about communication between the user (or users) and the computer. Therefore, the computer needs to be able to communicate to you, that is, send output. And you need to be able to communicate to the computer, that is, provide input.

print()

You can print a message to the standard output with the print() function. (print() accepts all the variable types that we will discuss below.)

print('Hello world!')
print(10)
print(True)
print(None)

Output:

Hello world!
10
True
None

The standard output is the default output channel of your program. If you're using Python from a terminal, then whatever goes to the standard output appears in the terminal; if you're using Python from within Rapunzel or Spyder, then whatever goes to the standard output appears in the IPython console; if you're using Python from within a Jupyter Notebook, then whatever goes to the standard output appears in the notebook; an in the interactive mini exercises, whatever goes to the standard output appears below the exercise.

But you generally don't have to worry about this: Your output will appear where you expect it to appear.

input()

You can read text from the standard input with the input() function. (This returns a str that you can assign to a variable, as we will discuss below.)

user_input = input('>>> ')
print(user_input)

Output:

>>> Hello world!
Hello world!

The standard input is the default input channel of your program. If you're using Python in an interactive environment, such as this website, Rapunzel, Spyder, or a Jupyter Notebook, then the user is prompted to provide input whenever the program tries to read from the standard input.

Again, you generally don't have to worry about this: User input will be handled as you'd expect it to.

Comments

A line that starts with a # is a comment. Comments only serve to document your code. It is good practice to comment code that is not self-explanatory. However, in many cases, well-written code is self-explanatory, and adding (too many) comments will only make it less clear.

# This is a comment!

Variable assignment

Python is dynamically typed, which means that you not need to explicitly indicate whether a variable is a number, a string, or something else. That doesn't mean that there are no variable types: There are, but the type of a variable is determined implicitly by the value that you assign (=) to it. (We will meet different variable types below.)

a = 1
print(a)

Output:

1

You can also assign a single value to multiple variables in a single statement. And you can even assign multiple values to multiple variables in a single statement:

b = c = 2 # b = 2; c = 2
d, e = 3, 4 # d = 3; e = 4

It is good practice to use variable names that are descriptive but not too long. For example, if a variable contains someone's age, the name age is preferable to a or (even worse) x. Variable names should be lowercase with words optionally separated by underscores, like_this. (Naming conventions such as this are described in the PEP8 style guideline.)

Constants

A constant is a computer-science term for a variable that has a fixed value. In Python, real constants don't exist, because you can always change a variable. However, to indicate that a variable should be treated as if it were a constant, you can capitalize its name, LIKE_THIS.

# PI should be treated as a constant
PI = 3.14

Common variable types

bool: True or False

A bool (from Boolean) is a variable type with only two values: True or False. It is used for logical operations including, as you will see below, if statements. Many operations, such as >, ==, and !=, result in a bool. There is also a special set of Boolean operations: and, or, and not

print(4 > 3)
print(3 > 4)
print(3 == 4) # The equality operator is a double equals sign!
print(4 > 3 or 3 > 4)
print(4 > 3 and not 3 > 4)

Output:

True
False
False
True
True

str: text strings

Text strings (str) are indicated by putting text between single (') or double (") quotes. You can specify strings spanning multiple lines with triplets of single (''') or double (""") quotes:

a_str = 'some text'
another_str = "some text"
a_multiline_str = '''Line 1
Line 2'''
another_multiline_str = """Line 1
Line 2"""

If you want to include quotes as characters in a string, you can 'escape' them using an \ to indicate that the character is part of the string.

an_escaped_str = 'You can escape \'quotes\' to make them part of the string'

You often want to print out a string of text with some values inserted into it, and there are several ways to do this. You can:

• Concatenate (+) strings.
• Use the % string-formatting operator, which trats the string as a template and inserts values into it.
• Use str.format(), which similarly treats the string as a template, and inserts values into it.
• Use f-strings (formatted string literals), which are prefixed with an f, and which allow you to embed variables directly in the string.

name = 'Audrey Tautou'
print('My name is ' + name)
print('My name is %s' % name)
print('My name is {0}'.format(name))
print(f'My name is {name}')  # This is what we will use

Output:

My name is Audrey Tautou
My name is Audrey Tautou
My name is Audrey Tautou
My name is Audrey Tautou

In addition to str.format(), str objects support many more functions. For example, the str.startswith() function checks whether the first part of a str is equal to another str.

print(another_str.startswith('some'))

Output:

True

For a complete list of str functions, see:

• https://docs.python.org/3/library/stdtypes.html#string-methods

None: a special kind of nothing

None is a special kind of value that is a type of its own (NoneType). Its main function is to indicate that a value is missing. To check whether a value is None, the special is operator is often used (and considered more elegant) instead of the regular == comparison.

age_depp = None
if age_depp is None:
    print('Johnny Depp is ageless')

Output:

Johnny Depp is ageless

int and float: numbers

The two main numeric types are:

• int for integer numbers (without decimals)
• float for floating-point numbers (with decimals)

Python automatically determines whether a value is an int or a float.

my_int = 7
my_first_float = 3.14
my_second_float = 7. # The decimal point indicates that this should be a float

You can perform all standard mathematical operations, such as + and *. You can determine the order in which operations should be evaluated with parentheses.

mul_add = 2 * 3 + 4
add_mul = 2 * (3 + 4)

# Print out some information about the numbers
print('First multiply then add: {0}'.format(mul_add))
print('First add then multiply: {0}'.format(add_mul))

Output:

First multiply then add: 10
First add then multiply: 14

The float type has two special values:

• nan is not a number and is generally used to indicate missing data in numeric computation. It is conceptually similar to None, with two important differences: nan is the only value that is unequal to itself (whereas None is equal to itself), and nan can be used in mathematical operations (whereas None cannot).
• inf is used to express infinite numbers. You will rarely encounter inf, unless you're doing specific kinds of mathematical computations.

Converting to float, int, or str

To convert a variable to a float, int, or str, you can use the corresponding float(), int(), and str() functions. A common use case is when you use input() to get a number from the user: input() returns a str, and you need to use int() or float() to convert the input to a number.

age = input('How old are you? ')
age = int(age)
if age >= 18:
    print('You are 18 years or older')
else:
    print('You are younger than 18')

Output:

How old are you? 35
You are 18 years or older

Converting values to another type is dangerous! If you try to convert the value 'this is not a number' to an int, you will get an error message, a so-called Exception. We discuss how to deal with Exceptions later

age = int('this is not a number')

Output:

ValueError: invalid literal for int() with base 10: 'this is not a number'

If you're unsure what type a variable has, you can check with type():

int_like_str = '10'
var_type = type(int_like_str)
print('int_like_str is a {var_type}')

Output:

int_like_str is a {var_type}

If statements

An if statement is the most basic form of flow control. It allows you to check whether some condition is True, and if so do something. If the condition is False, either nothing is done (if there is no else block), or something else is done. If multiple conditions are checked after each other, you can use the elif shorthand, which stands for else if:

# Without else
if 2 * 2 == 4:
    print('Two times to equals four')
# With else
if 1 * 1 == 2:
    print('This is not printed')
else:
    print('One times one does not equal two')
# With elif
if 3 * 3 == 8:
    print('This is not printed')
elif 3 * 3 == 9:
    print('Three times three equals nine')
else:
    print('This is not printed')

Output:

Two times to equals four
One times one does not equal two
Three times three equals nine

Here we also see an important aspect of Python syntax: colons and indentation. The if, elif, and else statements are followed by a colon (:). This indicates that a block of code will follow that is indented, typically by four spaces (as prescribed by PEP8) or a single tab. (But never mix different indentation styles in the same code!) The block ends when the indentation ends. Unlike many other programming languages, Python does not use curly braces or other ways to explicitly indicate the start and end of a block: blocks are defined entirely through indentation.

Review exercise

You're done with this section!