Hi Pythonistas!
As a developer, we all make mistakes while coding. Some of the mistakes are very silly and it takes a lot of effort while debugging. We are planning a series of blogs related to the common mistakes that I have made and explain why we should avoid them while coding. This is part 1 of this series.
Use of import *
Most of us use import * due to our laziness. But sometimes import * cause big issues and it takes a lot of time to debug this issue. I will explain this with an example.
I have 4 modules namely first.py, second.py, third.py, fourth.py
first.py
class MyClass(object):
def __init__(self, name):
self.name = name
def __str__(self):
return f"MyClass from first file: {self.name}"second.py
class MyClass(object):
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
return f"MyClass: {self.name} with age: {self.age}"third.py
from first import MyClass
class MyClassExtended(MyClass):
def __str__(self):
return f"Extended class with name: {self.name}"fourth.py
from second import MyClass
from third import *
print("\nI am creating my class extended\n")
my_ex = MyClassExtended("extendedclass")
print(my_ex)
print("\nI am Expecting MyClass is from second.py\n")
print("Actually my class is: ", MyClass, '\n')
my_class_2 = MyClass("Name", 2)
print(my_class_2)When we see this code, MyClass is imported from the second module, and it has two parameters. We are initializing the class correctly. But if we run this code, output will be an exception like this
output
afsal@afsal-Mi-NoteBook-Horizon-Edition-14:~/Desktop/experiments/import_experiment$ python3 fourth.py
I am creating my class extended
Extended classs with name: extendedclass
I am Expecting MyClass is from second.py
Actually my class is: <class 'first.MyClass'>
Traceback (most recent call last):
File "fourth.py", line 16, in <module>
my_class_2 = MyClass("Name", 2)
TypeError: __init__() takes 2 positional arguments but 3 were given
afsal@afsal-Mi-NoteBook-Horizon-Edition-14:~/Desktop/experiments/import_experiment$
Reason:
In fourth module we are importing * from third module and in third module we already imported MyClass from first module. Import * will copy the entire scope. As the statement “from second import MyClass” is above the "from third import *" statement, the current scope of fourth module has MyClass which is from first module. MyClass from second module is overwritten in this case. These kinds of errors are very hard to debug as the size of the repository is very big.
corrected.py
from second import MyClass
from third import MyClassExtended
print("\nI am creating my class extended\n")
my_ex = MyClassExtended("extendedclass")
print(my_ex)
print("\nI am Expecting MyClass is from second.py\n")
print("Actually my class is: ", MyClass, '\n')
my_class_2 = MyClass("Name", 2)
print(my_class_2)
Output:
afsal@afsal-Mi-NoteBook-Horizon-Edition-14:~/Desktop/experiments/import_experiment$ python3 correct.py
I am creating my class extended
Extended classs with name: extendedclass
I am Expecting MyClass is from second.py
Actually my class is: <class 'second.MyClass'>
MyClass: Name with age: 2
afsal@afsal-Mi-NoteBook-Horizon-Edition-14:~/Desktop/experiments/import_experiment$ Always import what you need.