Python Basics, IPython & Jupyter

Concept

• Python is an interpreted language: the interpreter runs your program one statement at a time.
• Type python in a terminal for the standard prompt (>>>); type exit() or press Ctrl-D (Ctrl-Z then Enter on Windows) to leave.
• Run a whole file with python hello_world.py. Inside IPython, %run hello_world.py does the same and keeps its variables afterwards so you can inspect them.
• Data analysts mostly live in IPython or Jupyter rather than the plain prompt — they add history, completion and rich output.

Concept

• IPython prints the result of the last expression in a cell automatically, numbered Out[n] to match In[n] — no print() needed.
• A Jupyter notebook is a document of cells; the kernel behind it keeps every variable alive between cells until you restart it.
• Tab completion: type part of a name, or obj., and press Tab to see matching variables, methods and even file paths.
• Introspection: obj? shows the type, value and docstring; func?? also shows the source code. np.*load*? searches names with wildcards.
• Cells run in the order you run them, not the order they appear — the numbers in In[n] tell you the real history. Restart and run all before trusting a notebook.

Concept

• Python uses indentation, not braces, to group code — four spaces is the convention. A colon ends the line before an indented block.
• Everything is an object: numbers, strings, lists, functions and modules all carry a type and live in memory as their own thing.
• Assignment makes a reference, not a copy: after b = a, both names point at the same list, so changing it through one is visible through the other. Function arguments are passed the same way.
• Types are dynamic (a name can point at anything) but strong: "5" + 5 raises TypeError instead of guessing. Check types with isinstance(x, (int, float)).
• Duck typing: care what an object can do, not what it is — "if it walks like a duck…". A function that accepts any iterable is more useful than one that demands a list.
• is tests identity (same object); == tests equality (same value). Lists and dicts are mutable; strings and tuples are immutable.

Concept

• int can hold arbitrarily large numbers; float is double precision. / always gives a float (3 / 2 is 1.5); // is floor division.
• First-edition trap: in Python 2 (what my old copy used) 3 / 2 was 1. If an old example gives odd numbers, check its division.
• str is an immutable sequence of Unicode characters — you can slice it but not assign into it. f-strings format values: f"{amount:.2f} {currency}".
• bytes are raw encoded data; .encode("utf-8") turns a string into bytes and .decode("utf-8") turns them back.
• bool is True/False (with and, or, not); None is the "no value" value — functions without a return give None.
• The datetime module gives date, time and datetime. strftime formats a date into text; strptime parses text into a date.

Concept

• if / elif / else checks conditions in order and runs only the first true branch. and/or short-circuit: the right side is skipped if the left decides the answer.
• for x in collection: loops over any iterable. continue skips to the next item; break leaves the innermost loop.
• while repeats until its condition becomes false — make sure something inside changes it.
• pass is the "do nothing" statement, a placeholder for code you haven't written yet.
• range(start, stop, step) produces evenly spaced integers lazily, stopping before stop. Later chapters replace most of these loops with array operations.