7.3 Top-level code
Learning objectives
By the end of this section you should be able to
Identify code that will run as a side effect of importing. Explain the purpose of if __name__ == "__main__".
Side effects
Modules define functions and constants to be used in other programs. When importing a module, all code in the module is run from top to bottom. If a module is not designed carefully, unintended code might run as a side effect. The unintended code is generally at the top level, outside of function definitions.
Using __name__
Python modules often include the statement if __name__ == "__main__" to prevent side effects. This statement is true when the module is run as a program and false when the module is imported.
Note
Exploring further
Variables that begin and end with double underscores have special meaning in Python. Double underscores are informally called "dunder" or "magic" variables. Other examples include __doc__ (the module's docstring) and __file__ (the module's filename).
Note
Side effects
This exercise is a continuation of the "Missing imports" exercise. Previously, you added missing import statements to the top of the program. Now, modify the program to prevent side effects when importing the program as a module:
Add if __name__ == "__main__" at the end. Move all test code under that if statement. The program should run without errors and produce the same output as before.
from math import sqrt
from area import circle
def is_prime(n):
"""Returns True if n is prime."""
if n < 2:
return False
for i in range(2, int(sqrt(n)) + 1):
if n % i == 0:
return False
return True
# Test the is_prime function
for n in range(10):
if is_prime(n):
print(n, "is prime")
else:
print(n, "is NOT prime")
def pizza_rate(size, price):
"""Calculates the value of a pizza coupon.
Args:
size (int): Diameter of the pizza in inches.
price (float): Advertised price of the pizza.
Returns:
float: The pizza's price per square inch.
"""
return price / circle(size / 2)
# Test the pizza_rate function
print()
print(" Small for $4.99:", pizza_rate(10, 4.99))
print(" Medium for $5.99:", pizza_rate(12, 5.99))
print(" Large for $7.99:", pizza_rate(14, 7.99))
print("X-Large for $9.99:", pizza_rate(16, 9.99))
Note
Conversion test
This exercise is a continuation of the "Conversion module" exercise. Previously, you wrote the functions cel2fah, fah2cel, km2mi, and mi2km. Write test code at the end of conversion.py (the original file) for each of these functions. The test code must not run as a side effect when conversion is imported by other programs. When running conversion.py as the main program, the test output should be:
0 C is 32 F
5 C is 41 F
10 C is 50 F
15 C is 59 F
20 C is 68 F
20 F is -7 C
25 F is -4 C
30 F is -1 C
35 F is 2 C
40 F is 4 C
1 km is 0.6 mi
2 km is 1.2 mi
3 km is 1.9 mi
4 km is 2.5 mi
5 km is 3.1 mi
5 mi is 8.0 km
6 mi is 9.7 km
7 mi is 11.3 km
8 mi is 12.9 km
9 mi is 14.5 km"""Functions that convert metric and imperial units."""
def cel2fah(c):
"""Converts from Celsius to Fahrenheit."""
return 9/5 * c + 32
def fah2cel(f):
"""Converts from Fahrenheit to Celsius."""
return 5/9 * (f - 32)
def km2mi(km):
"""Converts from kilometers to miles."""
return km / 1.60934
def mi2km(mi):
"""Converts from miles to kilometers."""
return mi * 1.60934 