Programmier-Werkstatt: Python: Key Points

(ProgWerk) Python Fundamentals

Basic data types in Python include integers, strings, and floating-point numbers.
Use variable = value to assign a value to a variable in order to record it in memory.
Variables are created on demand whenever a value is assigned to them.
Use print(something) to display the value of something.
Use # some kind of explanation to add comments to programs.
Built-in functions are always available to use.

[value1, value2, value3, ...] creates a list.
Lists can contain any Python object, including lists (i.e., list of lists).
Lists are indexed and sliced with square brackets (e.g., list[0] and list[2:9]), in the same way as strings and arrays.
Lists are mutable (i.e., their values can be changed in place).
Strings are immutable (i.e., the characters in them cannot be changed).

Use for variable in sequence to process the elements of a sequence one at a time.
The body of a for loop must be indented.
Use len(thing) to determine the length of something that contains other values.

Use if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.
The bodies of the branches of conditional statements must be indented.
Use == to test for equality.
X and Y is only true if both X and Y are true.
X or Y is true if either X or Y, or both, are true.
Zero, the empty string, and the empty list are considered false; all other numbers, strings, and lists are considered true.
True and False represent truth values.

Define a function using def function_name(parameter).
The body of a function must be indented.
Call a function using function_name(value).
Numbers are stored as integers or floating-point numbers.
Variables defined within a function can only be seen and used within the body of the function.
Variables created outside of any function are called global variables.
Within a function, we can access global variables.
Variables created within a function override global variables if their names match.
Use help(thing) to view help for something.
Put docstrings in functions to provide help for that function.
Specify default values for parameters when defining a function using name=value in the parameter list.
Parameters can be passed by matching based on name, by position, or by omitting them (in which case the default value is used).
Put code whose parameters change frequently in a function, then call it with different parameter values to customize its behavior.

Tracebacks can look intimidating, but they give us a lot of useful information about what went wrong in our program, including where the error occurred and what type of error it was.
An error having to do with the ‘grammar’ or syntax of the program is called a SyntaxError. If the issue has to do with how the code is indented, then it will be called an IndentationError.
A NameError will occur when trying to use a variable that does not exist. Possible causes are that a variable definition is missing, a variable reference differs from its definition in spelling or capitalization, or the code contains a string that is missing quotes around it.
Containers like lists and strings will generate errors if you try to access items in them that do not exist. This type of error is called an IndexError.
Trying to read a file that does not exist will give you an FileNotFoundError. Trying to read a file that is open for writing, or writing to a file that is open for reading, will give you an IOError.

Program defensively, i.e., assume that errors are going to arise, and write code to detect them when they do.
Put assertions in programs to check their state as they run, and to help readers understand how those programs are supposed to work.
Use preconditions to check that the inputs to a function are safe to use.
Use postconditions to check that the output from a function is safe to use.
Write tests before writing code in order to help determine exactly what that code is supposed to do.

The sys library connects a Python program to the system it is running on.
The list sys.argv contains the command-line arguments that a program was run with.
Avoid silent failures.
The pseudo-file sys.stdin connects to a program’s standard input.