Python Syntax#

This short tutorial is a very brief exploration of the essential features of Python syntax. It is mainly based on content from the App Tinkerstellar by Alex Staravoitau and Jack VanderPlas’s Whirlwind Tour of Python.

  • Syntax refers to the structure of the language (i.e., what constitutes a correctly-formed program).

  • Here, we’ll not focus on the semantics – the meaning of the words and symbols within the syntax.

# set the midpoint
midpoint = 5

# make two empty lists
lower = []
upper = []

# split the numbers into lower and upper
for i in range(10):
    if (i < midpoint):
        lower.append(i)
    else:
        upper.append(i)
        
print("lower:", lower)
print("upper:", upper)

lower: [0, 1, 2, 3, 4]
upper: [5, 6, 7, 8, 9]

Comments#

  • Comments in Python are marked by #

  • Anything on the line following the # sign is ignored by the interpreter

# set the midpoint

# below are examples of inline comments
x = 1 # create a variable x and assign value 1
x += 2  # shorthand for x = x + 2
print(x) # print x

3

Statements#

  • End-of-line terminates a statement in Python

midpoint = 5

  • This is an assignment operation, where we’ve created a variable named midpoint and assigned it the value 5.

  • Notice that the end of this statement is simply marked by the end of the line.

  • If you’d like a statement to continue to the next line, it is possible to use the “" marker to indicate this:

x = 1 + 2 + 3 + 4 +\
    5 + 6 + 7 + 8
print(x)

36

  • It is also possible to continue expressions on the next line within parentheses:

x = (1 + 2 + 3 + 4 +
     5 + 6 + 7 + 8)
print(x)

36

  • Most Python style guides recommend the second version of line continuation (within parentheses)

Indentation#

  • A block of code is a set of statements that should be treated as a unit

  • Code blocks are denoted by indentation (white space)

  • The amount of whitespace used for indenting code blocks is up to the user, as long as it is consistent throughout the script

  • By convention, most style guides recommend to indent code blocks by four spaces

  • Indented code blocks are always preceded by a colon (:) on the previous line

midpoint = 5
for i in range(10):
    if i < midpoint:
        lower.append(i)
    else:
        upper.append(i)

  • In the snippet below, print(x) is in the indented block, and will be executed only if x is less than 4

x=5
if x < 4:
    y = x * 2
    print(x)

  • In the snippet below, print(x) is outside the block, and will be executed regardless of the value of x

x=5
if x < 4:
    y = x * 2
print(x)      

5

Whitespace within lines#

  • White space within lines of Python code does not matter

  • For example, all three of these expressions are equivalent

a=1+2
a = 1 + 2
a   =   1   +   2

print(a)

3

  • Using whitespace effectively can lead to much more readable code

b=10**-2

b = 10 ** -2

Parentheses#

  • Parentheses are used for grouping or calling

  • They can be used in the typical way to group statements or mathematical operations:

2 * (3 + 4)

14

  • They can also be used to indicate that a function is being called.

  • In the next snippet, the print() function is used to display the contents of a variable.

  • The function call is indicated by a pair of opening and closing parentheses, with the arguments to the function contained within:

print('Value:', 1)

Value: 1

  • Some functions can be called with no arguments at all, in which case the opening and closing parentheses still must be used to indicate a function evaluation.

  • An example of this is the sort method of lists:

L = [4,2,3,1]
L.sort()

print(L)

[1, 2, 3, 4]

  • The “()” after sort indicates that the function should be executed, and is required even if no arguments are necessary.

Style guides#