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Python Notes

Python notes

Running Python Scripts

Can do python... file_name.py to run a file, or can open a REPL in the terminal by just writing python... or can just click start button.

F5 to debug.

The -i flag means ‘open a REPL that can use the code in this file’ - e.g. python -i 017_expressions.py in the terminal, either iTerm or in VSCode Terminal

can exit REPL by writing exit() or using ctrl-d

indentation is important!

F strings

e.g. f"hello {name}" The python version of template literals in js.

Type

The equivalent of typeof in python is type().

New lines

\n - new line in python.

Snake case

Python generally uses snake case for variables - e.g. snake_case

Using Translation table

Have to set the translation table and then use the translate function

my_string = "tamperlot"
translation_table = str.maketrans({'t': 'c', 'l': 'b'})
translated_string = my_string.translate(translation_table)

Functions

Remember to write def when writing a function, e.g. def my_function(): and have import modules at the top of the file, outside the main function. Use ifname==“main”: to run the function and not the contents of the import modules.

def my_function():
    print("hello")
    print("how are you?")


if __name__ == "__main__":
    my_function()

Comments

# is a comment in python """ or ''' is a multiline comment

Single underscore

_ is a single underscore and is used for private variables you don’t care about in python.

List comprehensions

Help make the code more efficient, with less syntax. Can mimic certain lambda functions, easier to read. - e.g.

my_list = [x*x for x in range(10)]

instead of

my_list = [] for i in range(1,11): my_list.append(x*x)`

can also use if in the list comprehension:

list = [expression for item in iterable if conditional]

as well as:

list = [expression for item in iterable] list = [expression if/else for item in iterable]

NB the expression and everything has to be inside the [] for the list.

all function

all() is a built-in Python function that returns True if all the elements inside a given iterable evaluate to True. Otherwise, it returns False.

Can be added to if statement to check if all elements in a list are true.

Generator expression

my_list = (x*x for x in range(10))

Just like list comprehension, but without square brackets. Can be faster, and uses less memory as it is not stored but deleted after use.

Keyword arguments

These are used to pass arguments to functions. e.g. my_function(1, 2, 3) becomes my_function(a=1, b=2, c=3)

  • When you use keyword arguments, the order of the arguments does not matter.
  • You can use default values if you don’t want to pass an argument.
  • You can use a mix of positional and keyword arguments.
  • You can use just one keyword argument to pass the default value of other arguments.

## range function NB range is a built-in function in python and can be used with for loops, e.g. for i in range(number_of_moves):

NB negation in python

In Python, the not keyword is used for negation, whereas ! is not a valid operator for negation in Python. Different to js. Nevertheless, not equal (!=) is still valid.

NB

Have to write += because python prefers to be explicit and not use ++ .

use .extend() instead of += for lists

row_empty = row.replace(‘0’, ’ ’) replace (x, y) where x replaces y.

Inside the method, unpack the empty tuple into row and col.

row, col = empty

Walrus operator

the walrus operator (:=), otherwise known as the assignment expression operator, combines the assignment and the conditional check into a single line, e.g.

if (next_empty := self.find_empty_cell()) is None

Classes

Define a class with the class keyword, e.g. class MyClass:

Roadmap

Fill this in as learn: https://roadmap.sh/python

Resources

Python documentation: https://docs.python.org/3/tutorial/datastructures.html

https://fireship.io/lessons/code-this-not-that-python-edition/