Next, we will learn how to make choices in our code using conditional statements (
else) and Boolean values (
Basics of conditional statements#
Conditional statements can change the code behaviour based on certain conditions. The idea is simple: IF a condition is met, THEN a set of actions is performed.
A simple conditional statement#
Let’s look at a simple example with temperatures, and check if temperature 17 (celsius degrees) is hot or not:
temperature = 17 if temperature > 25: print("it is hot!") else: print("it is not hot!")
it is not hot!
What did we do here? First, we used the
else statements to determine what parts of the code to execute. The
if statement checks to see whether the variable value for
temperature is greater than 25. If this condition is met,
'it is hot' would be written to the screen. Since 17 is smaller than 25, the code beneath
else is executed. Code under the
else statement will run whenever the
if test is
temperature to a “hot” temperature and repeat the same process:
temperature = 30 if temperature > 25: print("it is hot!") else: print("it is not hot!")
it is hot!
The code indented under the if-statement is not executed if the condition is not True. Instead, code under the else-statement gets executed.
else? The combination of
else is very common, but the
else statement is not strictly required. Python simply does nothing if the
if statement is False and there is no
temperature = 17 if temperature > 25: print(temperature, "is greater than 25")
Makes sense, right? Conditional statements always check if the conditional expression is True or False. If True, the codeblock under the conditional statement gets executed. Nothing is printed to the screen if temperature is smaller than 25.
Let’s look at another example from our daily lives. As it turns out, we all use logic similar to
else conditional statements daily. Imagine you’re getting ready to leave your home for the day and want to decide what to wear. You might look outside to check the weather conditions. If it is raining, you will wear a rain jacket.
Otherwise, you will not. Remember that Python uses the
== operator to test if a value is exactly equal to another.
weather = "rain" if weather == "rain": print("Wear a raincoat!") else: print("No raincoat needed.")
Wear a raincoat!
Similarly as with for loops, Python uses colons (
:) and whitespace (indentations; often four spaces) to structure conditional statements. If the condition is
True, the indented code block after the colon (
:) is executed. The code block may contain several lines of code, but they all must be indented identically You will receive an
SyntaxError, or unwanted behavior if you haven’t indented your code correctly.
Note also that the case of the text being compared (uppercase or lowercase) is important. For instance, in the example above, if we define
weather = 'Rain', the comparsion
weather == 'rain' would be false. One possible solution to this problem is to use the
.lower() method for strings, which would convert the text to which it is applied to lowercase. In the example here, if we define
weather = Rain, the comparison
weather.lower() == 'rain' would be true!
We might also need some other rainwear on a rainy day. Think about how you could add another instruction after the
weather == rain condition so that the code would tell us to:
Wear a raincoat Wear rain boots
# Solution weather = "rain" if weather == "rain": print("Wear a raincoat") print("Wear rain boots") else: print("No rainwear needed")
Wear a raincoat Wear rain boots
Comparison operators such as
== compare the values on each side of the operator. Table 2.3 lists operators used for value comparisons in Python:
: Table 2.3. Comparison operators in Python.
Less than or equal to
Greater than or equal to
Not equal to
Comparison operations yield boolean values (
False). In Python, the words
False are reserved for these Boolean values, and can’t be used for anything else.
Let’s check the current value of the conditions we used in the previous examples:
temperature > 25
weather == "rain"
if, elif and else#
We can link several conditions together using the “else if” -statement
elif. Python checks the
else statements only if previous conditions were
False. You can have multiple
elif statements to check for additional conditions. Let’s create a chain of
else -statements that are able to tell us if the temperature is above freezing, exactly at freezing point or below freezing:
temperature = -3
if temperature > 0: print(temperature, "degrees celsius is above freezing") elif temperature == 0: print(temperature, "degrees celsius is at the freezing point") else: print(temperature, "degrees celsius is below freezing")
-3 degrees celsius is below freezing
Let’s assume that yesterday it was 14°C, it is 10°C outside today, and tomorrow it will be 13°C. The following code compares these temperatures and prints something to the screen based on the comparison.
yesterday = 14 today = 10 tomorrow = 13 if yesterday <= today: print('A') elif today != tomorrow: print('B') elif yesterday > tomorrow: print('C') elif today == today: print('D')
Which of the letters
D would be printed out?
# Solution "B"
We can also use
or to combine multiple conditions on boolean values (Table 2.4).
: Table 2.4. Logic for the
or keywords in Python.
True if both
True if either
if (1 > 0) and (-1 > 0): print("Both parts are true") else: print("At least one part is not true")
At least one part is not true
if (1 < 0) or (-1 < 0): print("At least one test is true")
At least one test is true
Later in this book we will also need the bitwise operators
Let’s return to our example about making decisions on a rainy day. Imagine that we consider not only the rain, but also the wind speed (in meters per second). If it is windy or raining, we’ll just stay at home. If it’s not windy or raining, we can go out and enjoy the weather!
Let’s set 10 m/s as our comfort limit in the conditional statement and see what our Python program tells us to do in these conditions:
# Solution weather = "rain" wind_speed = 14 comfort_limit = 10 # If it is windy or raining, print "stay at home", # otherwise (else) print "go out and enjoy the weather!" if (weather == "rain") or (wind_speed >= comfort_limit): print("Just stay at home") else: print("Go out and enjoy the weather! :)")
Just stay at home
As you can see, we better just stay home if it is windy or raining! If you don’t agree, you can modify the conditions and print statements accordingly.
Combining for-loops and conditional statements#
Finally, we can also combine for-loops and conditional statements. Let’s iterate over a list of temperatures, and check if the temperature is hot or not:
temperatures = [0, 12, 17, 28, 30] # For each temperature, if the temperature is greater than 25, print "..is hot" for temperature in temperatures: if temperature > 25: print(temperature, "is hot") else: print(temperature, "is not hot")
0 is not hot 12 is not hot 17 is not hot 28 is hot 30 is hot