from operator import add, mul
from operator import *

Though you can import everything from math, or operator, it’s more common to specify. Thus,

from math import pi, sin, cos
Binding two names at once
area, circ = pi * radius * radius, 2 * pi * radius
We can bind the build-in function names to values
>>> max(3, 4)
>>> max = 7
>>> max(4, 5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable

Now that we’ve reassigned our very useful max function to just a number, we could get it back with:

__builtins__.max(3, 4)
max = __builtins__.max

or, more simply,

del max


An expression describes a computation and evaluates to a value.

Primitive expressions

A primitive expression is a single evaluation step. You either look up the value of a name or take the literal value. Some examples are:

Expression Type
2 number/numeral
'hello' string
add name
Call expressions

A call expression makes one or more function calls, for example:

max(1, 2)

Environment Diagrams


  • Within a frame, a name cannot be repeated
  • Every time a user-defined function is called, a local frame is created

Looking up Names in Environments

The current environment is either:

  • the global frame alone, or
  • a local frame, followed by the global frame

Defining Functions

Assignment is a simple means of abstraction, by binding names to values

Function definition is a more powerful means of abstraction, by binding names to expressions

def <name>(<formal parameters>):
    return <return expression>

A function signature indicates how many arguments a function takes.

A function body defines the computation performed when the function is applied.

Hey, kudos for making it this far! If you've liked this, you might also like tmux Cheatsheet and Shortcuts.