Idiomatic Python - miscellaneous part 2

String concatenation

In [1]:
names = ('John', 'Lisa', 'Terminator', 'Python')

Don't do this.

In [2]:
semicolon_separated = names[0]
for name in names[1:]:
    semicolon_separated += ';' + name
print(semicolon_separated)

John;Lisa;Terminator;Python

Use `join` instead!

In [3]:
semicolon_separated = ';'.join(names)
print(semicolon_separated)

John;Lisa;Terminator;Python

or in assignments

The return value of a or b:

  • a if a is truthy
  • b otherwise

You can take advantage of this e.g. while writing variable assignments.

In [4]:
a = 0
b = None
c = 'John Doe'

Instead of doing something like this:

In [5]:
my_variable = 'default value'
if a:
    my_variable = a
elif b:
    my_variable = b
elif c:
    my_variable = c
print(my_variable)

John Doe

Prefer doing this:

In [6]:
my_variable = a or b or c or 'default value'
print(my_variable)

John Doe

try - except - else

Don't use the following technique for checking if there was exceptions during execution of some block of code.

In [7]:
exception_occured = False
try:
    # here would be the logic of your master piece
    
    bad_calculation = 1 / 0
    
except ValueError as e:
    print('Oh boi, some value error: {}'.format(e))
    exception_occured = True
except Exception as e:
    print('Oh boi, something bad happened: {}'.format(e))
    exception_occured = True
    
if not exception_occured:
    print('All went well!')

Oh boi, something bad happened: division by zero

Use this instead!

In [8]:
try:
    # here would be the logic of your master piece
    
    bad_calculation = 1 / 0
    
except ValueError as e:
    print('Oh boi, some keyerror: {}'.format(e))
except Exception as e:
    print('Oh boi, something bad happened: {}'.format(e))
else:
    print('All went well!')

Oh boi, something bad happened: division by zero

try - finally

For scenarios where you want to do something always, even when there are exceptions.

Don't do it like this

In [9]:
def magical_calculation():
    try:
        # here would be the logic of your master piece
        result = 1 / 0
    except ZeroDivisionError:
        print('This could be something important that should be done every time')
        return 0
    except Exception:
        print('This could be something important that should be done every time')
        return None

    print('This could be something important that should be done every time')
    return result

print('return value: {}'.format(magical_calculation()))

This could be something important that should be done every time
return value: 0

This is better fit for the purpose!

In [10]:
def magical_calculation():
    try:
        # here would be the logic of your master piece
        result = 1 / 0
    except ZeroDivisionError:
        return 0
    except Exception:
        return None
    finally:
        print('This could be something important that should be done every time')
    return result

print('return value: {}'.format(magical_calculation()))

This could be something important that should be done every time
return value: 0

Note: You can also have try-except-else-finally structure. In cases where exception is not raised inside try, else will be executed before finally. If there is an expection, else block is not executed.

Use context managers when possible

One use case example is file I/O.

Don't play with files like this.

In [11]:
try:
    some_file = open('tmp.txt', 'w')
    print('the file is now open: {}'.format(not some_file.closed))
    
    # here would be some logic
 
finally:
    some_file.close()
    print("now it's closed: {}".format(some_file.closed))

the file is now open: True
now it's closed: True

Use context manager instead!

In [12]:
with open('tmp.txt', 'w') as some_file:
    print('the file is now open: {}'.format(not some_file.closed))
    
    # here would be some logic

print("now it's closed: {}".format(some_file.closed))

the file is now open: True
now it's closed: True

It's also easy to implement one yourself.

In [13]:
from contextlib import contextmanager

@contextmanager
def my_context():
    print('Entering to my context')
    yield
    print('Exiting my context')
    
def do_stuff():
    with my_context():
        print('Doing stuff')
        
    print('Doing some stuff outside my context')
        
do_stuff()  

Entering to my context
Doing stuff
Exiting my context
Doing some stuff outside my context

min() & max()

In [14]:
secret_data = (1, 2, 5, 99, 8, -9)

No need to bake it yourself.

In [15]:
max_value = 0
for val in secret_data:
    if val > max_value:
        max_value = val
print(max_value)

99

Use builtin functionality instead!

In [16]:
max_value = max(secret_data)
print(max_value)

99

contextlib.suppress - ignoring exceptions

If there's a potential exception that is ok, don't handle it like this.

In [17]:
value = 0
try:
    value = 1 / 0  # just for demonstrating purposes 
except ZeroDivisionError:
    pass

print(value)

0

Do it like this instead!

In [18]:
from contextlib import suppress

value = 0
with suppress(ZeroDivisionError):
    value = 1 / 0  # just for demonstrating purposes
    
print(value)

0

Properties instead of getter/setter methods

Instead of doing something like this.

In [19]:
class Person:
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name
        
    def get_full_name(self):
        return '{} {}'.format(self.first_name, self.last_name)
    
    def set_full_name(self, full_name):
        parts = full_name.split()
        if len(parts) != 2:
            raise ValueError('Sorry, too difficult name')
            
        self.first_name, self.last_name = parts 
        
      
p = Person('John', 'Doe')
print(p.get_full_name())
p.set_full_name('Lisa Doe')
print(p.get_full_name())

John Doe
Lisa Doe

Prefer properties!

In [20]:
class Person:
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name
        
    @property
    def full_name(self):
        return '{} {}'.format(self.first_name, self.last_name)
    
    @full_name.setter
    def full_name(self, name):
        parts = name.split()
        if len(parts) != 2:
            raise ValueError('Sorry, too difficult name')
            
        self.first_name, self.last_name = parts

    
p = Person('John', 'Doe')
print(p.full_name)
p.full_name = 'Lisa Doe'
print(p.full_name)

John Doe
Lisa Doe