Python language

python-language.md

Python Handbook

Intended audience: relatively experienced software engineers who don't know Python (yet).

Python lambdas

Python isn't a "functional" language, but it has adopted some FP methods around 1994.

A function in Python:

def identity(x):
   return x

A lambda function in Python (a unary arity anonymous function without parentheses):

lambda x: x

Here's another one.

something_terse=lambda t: t.property_thing

^ What's it doing? This expression is a lambda function which takes t as unary arg, returns t.property_thing, and assigns the returned value of t.property_thing to something_terse variable.

Python *args and **kwargs

*args: Variable arity arguments

• Also known as "non-keyword args"
• Similar to argc and argv in C
• Similar to the spread or rest operator ... in JavaScript
• Example:

  def a_func(arg1, *argv): # Note that there's no need to _actually_ call it *args. That's just an idiomatic nomenclature.
     print(arg1)
     for arg in argv:
        print("next arg:", arg)
   
   a_func("hello", "I", "liek", "catz") # prints each one. Variadic!

**kwargs: Variable arity collection (dict) of arguments with key-value pairs

• Example:

  def so_func(**kwargs):
     for key, val in kwargs.items:
        print("%s == %s &(key,val))
   
   so_func(first = "hello", mid = "I", last = "catz") # prints each

Python with keyword: context manager / context guard.

The Python with keyword class __enter()__ and __exit()__.

1. Python evaluates the expression;
2. Then calls __enter()__ on the resulting value (context guard);
3. Then assigns whatever __enter()__ returned to the variable given by as._
4. Then executes the code body;
5. Finally, no matter what has occurred, calls __exit()__ (context guard).

Python getattr() method:

Returns the value of a named attributed of an object.

If not found, returns the default value supplied.

getattr(object, name, [default_value])

Uses:

• The with keyword in Python is good for un-managed resources, like file streams
• The classic example is opening a file:

  with open('file.txt', 'w') as f:
  f.write('hi there!')
  ``

• Consider try/catch:

   # set things up:
   try:
     # do something
   finally:
     # teardown

Unit Testing in Python: UnitTest and Pytest

Pytest: native language features instead of special assertion methods like UnitTest

Test fixtures in Pytest can be shared by multiple test cases.

# decorator. Specify args to test function. 
# Should have same name that resource does
@pytest.fixture
def resource():
   return Resource()

UnitTest library

• .assertEqual(expected, actual)
• > python -m unittest : Execute Python Unit Test Module on the CLI

Exceptions: UnitTest

• self.assert : Raises method
• use context, with keyword:

  def test_missing_name(self):
  phonebook = PhoneBook()
  with self.assert Raises  (KeyError) ???
  phonebook.lookup("missing")

  • `