Flow control¶

When Python runs a script the code is executed from the top to the bottom. However, you might want to only run certain parts of the code given specific conditions, or you might want to repeat something multiple times. To do this you can use flow control:

if... elif... else statements (sometime called clauses)
for loops
while loops

Indenting¶

In Python the way to specify that you are defining something "inside" a flow control statement, or function, or class, is by using indenting. Most other languages use parentheses (i.e., brackets) of some sort, but in Python indenting is essential.

Things that are together within the same statement or definition must be indented using whitespace (regular spaces or tabs) to the same level.

Tip

Be consistent with your whitespace. It is highly recommended that you use spaces rather than tabs. Using four spaces for each level of indentation is considered standard and is recommended.

If you are nesting statements (i.e., flow control within a function definition, or loops within loops), each statement definition must be further indented.

if a == b:
    # first level of indentation (within if statement)
    for x in range(y):
        # second level of indenting (within for loop and if statement)
        if x > a:
            # third level of indenting (within if statement within for loop
            # within outer if statement)
            ...

Correct examples

x = True
if x is True:
    # Indented with 4 spaces
    print("x is true!")

def my_function(a):
    # first level of indentation
    x = range(a)
    y = []
    for i in x:
        # second level of indentation
        y.append(i * 2 + 1)

    # back to first level of indenting (i.e., outside the for loop, but still in the function definition)
    return y

# back outside the function definition
b = my_function(10)
print(b)

Incorrect examples

x = True
if x is True:
# we've forgotten any indentation!
print("x is True")
IndentationError: expected an indented block

if x is True:
  # different indenting levels
  print("x is True")
    print("...or is it!")
IndentationError: unexpected indent

If you are writing code in an IPython terminal session, or using VS Code with the Python extension, it can automatically indent for you. But, be careful!

Conditional expressions¶

Before discussing if statements we need to describe conditional expressions. These can be mathematical equalities and inequalities (useful for comparing numbers, but also usable for other objects):

x == y: "the value of x is equal to the value of y"
x != y: "the value of x is not equal to the value of y"
x > y: "the value of x is greater than the value of y"
x >= y: "the value of x is greater than, or equal to, the value of y"
x < y: "the value of x is less than the value of y"
x <= y: "the value of x is less than, or equal to, the value of y"

These return a boolean value of True or False:

x = 2
y = 3

x == y
False

x != y
True

x > y
False

x >= y
False

# save the output to a variable
expression = (x < y)
print(expression)
True

These expressions can be combined with the logical statements and, or, not:

x and y: True if both x is True and y is True
x or y: True if either x is True or y is True
not x: True if x is not True (i.e., x is False)

where x and y can be variables, or other conditional expressions like above. These expressions return boolean values:

a = 1
b = 5
c = 1

(a == c) and (a < b)
True

(a != c) or (a < b)
True

not (a == c)
False

Note

If you have an int or float variable that is set to zero it will evaluate as False in a logical expression while any non-zero value will evaluate as True. For array-like variables such as lists, dictionaries or tuples, or strings, an empty array will evaluate as False while an array containing any number of values will evaluate as True.

The is keyword can be used to test if two variables have the same value and are also of the same type:

x = 2     # integer value
y = 2.0   # float value

# test that they have the same value
x == y
True

# test that the are the same
x is y
False

`if` statements¶

You may want to execute a part of your code only if a certain condition is fulfilled. You can place that code inside an if clause:

x = 4

if x < 10:
    # indent the code within the if statement
    y = x * 2

# exit the if statement by removing the indent
print(y)
8

The condition being evaluated can be any combination of conditional expressions and logical statements (it is neater to have them each within brackets):

x = 5.5
y = "Hello"
z = 13

if ((x != 3.0) and (y[0] == "H")) or (x % 2):
    print("I'm in the if statement")
I'm in the if statement

`else`¶

If statements allow you to execute code under a certain condition, but what if you also want to execute some different code if the condition is not fulfilled? You can combine an if with an else:

x = 4

if x < 2:
    # indent inside the initial if statement
    print("My number is less than 2")
else:
    # indent again for the else statement
    print("My number is greater than or equal to 2")
"My number is greater than or equal to 2"

`elif`¶

There may be multiple exclusive conditions that you want to test for. To do this you can use elif (a shortening of "else if") statements after an initial if statement:

x = 3

if x < 2:
    # indent inside the initial if statement
    print("My number is less than 2")
elif x < 4:
    # indent again for the elif statement
    print("My number is greater than or equal to 2, but less than 4")
else:
    # indent again
    print("My number is greater than or equal to 4")
My number is greater than or equal to 2, but less than 4

You can have as many elif statements as required.

Note

You do not have to end with an else if you've covered all the condition that are required.

`for` and `while` loops¶

You often need to get code to repeat a task multiple times. Rather than having to repeatedly write the same piece of code over and over you can place it inside a for statement or while statement.

A for statement is used to increment though a sequence of values (i.e., those in a list or a tuple) and run whatever is within the statement at each iteration. The code within the for statement is often dependent on the current value in the sequence. Some basic examples are:

values = [0, 1, 2, 3, 4]
squaredvalues = []  # empty list that we'll append to
for i in values:
    # code within the loop must be indented
    print(i)
    squaredvalues.append(i ** 2)

# unindent to exit the loop code
print(squaredvalues)
0
1
2
3
4
[0, 1, 4, 9, 16]

sentence = ""
words = ["Coding", "in", "Python", "is", "fun!"]
for word in words:
    sentence += word + " "
print(sentence)
Coding in Python is fun!

The keywords used here are for and in. The value after in is the sequence to be iterated through; it can be any iterable, which in Python is any object that contains a set of values that can be moved through. The value between for and in will contain the current value from the sequence and can be used within the loop. It is a variable and can be named whatever you want it to be.

Useful `for` loop tips¶

`range`¶

The range built-in function is very useful in for loops to allow you to loop over a set of increasing or decreasing integer numbers. range can take either one, two or three integer value arguments:

# loop over the numbers 0 to 4 in steps to 1 (range goes from 0 to one less than the argument)
for i in range(5):
    print(i)
0
1
2
3
4

# loop over the numbers from 3 to 7 in steps of 1
for i in range(3, 8):
    print(i)
3
4
5
6
7

# loop over the numbers 2 to 10 in steps of 2
for i in range(2, 11, 2):
    print(i)
2
4
6
8
10

# loop backwards from 10 to 2 in steps of -2
for i in range(10, 1, -2):
    print(i)
10
8
6
4
2

Question

Why do you give it an integer one bigger than the last value? Because Python indexing starts at 0 and therefore this works:

x = [3, 1, 5, 7]
# use the length of x as the argument to range
for i in range(len(x)):
    print(x[i])
3
1
5
7

Note

In Python 3 range does not return a list, so if you want to use it to create a list you must do, e.g., x = list(range(10))

`enumerate`¶

The enumerate built-in function allows you to loop over both the indices and values of a sequence that you give it. For each iteration of a loop it returns a tuple pair containing the index and value (you can name these whatever you want), e.g.:

x = ["a", "b", "c"]
for i, xvalue in enumerate(x):
    # i contains the index, and x value contains the value
    print(i, xvalue)
0 a
1 b
2 c

`zip`¶

The zip built-in function allows you to "zip" together two or more equal length sequences. If used in a for loop it will return a tuple with the group's items, e.g.:

x = ["a", "b", "c"]
y = [12.4, 1.5, 6.7]
for xvalue, yvalue in zip(x, y):
    print(xvalue, yvalue)
a 12.4
b 1.5
c 6.7

`break` and `continue`¶

The break and continue keywords, combined with if...elif...else conditional statements, are ways of using flow control within a loop. break allows you to exit a loop if a certain condition is fulfilled, e.g.,

for i in range(-5, 6):
    # exit the loop if the numbers become positive
    if i >= 0:
        # indent again in the if statement
        break
    print(i)
-5
-4
-3
-2
-1

continue returns to the start of the loop without implementing any of the code below it within the loop, e.g.,:

for i in range(-5, 6):
    if i < 0:
        # do not reach the print statement if i is negative
        continue
    print(i)
0
1
2
3
4
5

`while` loops¶

A while loop takes a conditional expression and keeps looping over the code within it until that condition is satisfied:

x = 4
while x < 10:
    # indent the code within the loop
    x += 2
print(x)

Note

Even if the condition is fulfilled at the start of the loop the rest of the code below will still be run for that iteration:

x = 4
while x > 0:
    x -= 1
    print(x)
3
2
1
0

You can have "infinite" while loops by setting the condition to True, however these must contain a break statement otherwise they will never stop, e.g.,

x = 0
while True:
    if x > 10:
        # have a break statement within the loop
        break
    x += 1

List comprehension¶

You can use the for statement to create lists using a single line of code. This is called list comprehension. For example, suppose we had a list of values and we wanted to create a new list with the square of each of those values:

values = [1, 2, 3, 4, 5]
x = [y ** 2 for y in values]
print(x)
[1, 4, 9, 16, 25]

For reference, in case you see it in other code, another way of doing this is to use the map built-in function and a function (or a lambda function):

values = [1, 2, 3, 4, 5]
x = list(map(lambda x: x ** 2, values))
print(x)

Note

For ease of readability, among other things, it is highly recommended to use list comprehension rather than map.

We can also use an if statement within list comprehension if we require a specific condition to be met, e.g.:

import math
values = [-3, -2, -1, 0, 1, 2, 3]
# get a list of square roots, but only for positive numbers
x = [math.sqrt(y) for y in values if y > 0]
print(x)
[1.0, 1.4142135623730951, 1.7320508075688772]

There is a very similar construct for dictionaries, called dictionary comprehension:

values = [1, 2, 3]
keys = ["a", "b", "c"]
x = {keys[i]: values[i] for i in range(3)}
print(x)
{'a': 1, 'b': 2, 'c': 3}

Flow control¶

Indenting¶

Conditional expressions¶

if statements¶

else¶

elif¶

for and while loops¶

Useful for loop tips¶

range¶

enumerate¶

zip¶

break and continue¶

while loops¶

List comprehension¶

`if` statements¶

`else`¶

`elif`¶

`for` and `while` loops¶

Useful `for` loop tips¶

`range`¶

`enumerate`¶

`zip`¶

`break` and `continue`¶

`while` loops¶