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(f"Oh boi, some value error: {e}") exception_occured = True except Exception as e: print(f"Oh boi, something bad happened: {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(f"Oh boi, some keyerror: {e}") except Exception as e: print(f"Oh boi, something bad happened: {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(f"return value: {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(f"return value: {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(f"the file is now open: {not some_file.closed}") # here would be some logic finally: some_file.close() print(f"now it's closed: {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(f"the file is now open: {not some_file.closed}") # here would be some logic print(f"now it's closed: {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 f"{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 f"{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