reference.md

layout	title	subtitle
page	Programming with Python	Reference

Analyzing Patient Data

• Import a library into a program using import libraryname.
• Use the numpy library to work with arrays in Python.
• 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.
• The expression array.shape gives the shape of an array.
• Use array[x, y] to select a single element from an array.
• Array indices start at 0, not 1.
• Use low:high to specify a slice that includes the indices from low to high-1.
• All the indexing and slicing that works on arrays also works on strings.
• Use # some kind of explanation to add comments to programs.
• Use array.mean(), array.max(), and array.min() to calculate simple statistics.
• Use array.mean(axis=0) or array.mean(axis=1) to calculate statistics across the specified axis.
• Use the pyplot library from matplotlib for creating simple visualizations.

Repeating Actions with Loops

• Use for variable in collection to process the elements of a collection 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.

Storing Multiple Values in Lists

• [value1, value2, value3, ...] creates a list.
• Lists are indexed and sliced 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).

Analyzing Data from Multiple Files

• Use glob.glob(pattern) to create a list of files whose names match a pattern.
• Use * in a pattern to match zero or more characters, and ? to match any single character.

Making Choices

• Use the ImageGrid class from the ipythonblocks library to create simple "images" made of colored blocks.
• Specify colors use (red, green, blue) triples, each component of which is an integer in the range 0..255.
• 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.
• Nest loops to operate on multi-dimensional data.
• Put code whose parameters change frequently in a function, then call it with different parameter values to customize its behavior.

Creating Functions

• Define a function using def name(...params...).
• The body of a function must be indented.
• Call a function using name(...values...).
• Numbers are stored as integers or floating-point numbers.
• Integer division produces the whole part of the answer (not the fractional part).
• Each time a function is called, a new stack frame is created on the call stack to hold its parameters and local variables.
• Python looks for variables in the current stack frame before looking for them at the top level.
• 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).

Errors and Exceptions

• 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 if you use a variable that has not been defined (either because you meant to use quotes around a string, you forgot to define the variable, or you just made a typo).
• Containers like lists and dictionaries will generate errors if you try to access items in them that do not exist. For lists, this type of error is called an IndexError; for dictionaries, it is called a KeyError.
• Trying to read a file that does not exist will give you an IOError. Trying to read a file that is open for writing, or writing to a file that is open for reading, will also give you an IOError.

Defensive Programming

• 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.

Debugging

• Know what code is supposed to do before trying to debug it.
• Make it fail every time.
• Make it fail fast.
• Change one thing at a time, and for a reason.
• Keep track of what you've done.
• Be humble.

Command-Line Programs

• 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 "file" sys.stdin connects to a program's standard input.
• The "file" sys.stdout connects to a program's standard output.

Glossary

additive color model : A way to represent colors as the sum of contributions from primary colors such as red, green, and blue.

argument : A value given to a function or program when it runs. The term is often used interchangeably (and inconsistently) with parameter.

assertion : An expression which is supposed to be true at a particular point in a program. Programmers typically put assertions in their code to check for errors; if the assertion fails (i.e., if the expression evaluates as false), the program halts and produces an error message. See also: invariant, precondition, postcondition.

assign : To give a value a name by associating a variable with it.

body : (of a function): the statements that are executed when a function runs.

call stack : A data structure inside a running program that keeps track of active function calls.

case-insensitive : Treating text as if upper and lower case characters of the same letter were the same. See also: case-sensitive.

case-sensitive : Treating text as if upper and lower case characters of the same letter are different. See also: case-insensitive.

comment : A remark in a program that is intended to help human readers understand what is going on, but is ignored by the computer. Comments in Python, R, and the Unix shell start with a # character and run to the end of the line; comments in SQL start with --, and other languages have other conventions.

compose : To apply one function to the result of another, such as f(g(x)).

conditional statement : A statement in a program that might or might not be executed depending on whether a test is true or false.

comma-separated values : (CSV) A common textual representation for tables in which the values in each row are separated by commas.

default value : A value to use for a parameter if nothing is specified explicitly.

defensive programming : The practice of writing programs that check their own operation to catch errors as early as possible.

delimiter : A character or characters used to separate individual values, such as the commas between columns in a CSV file.

docstring : Short for "documentation string", this refers to textual documentation embedded in Python programs. Unlike comments, docstrings are preserved in the running program and can be examined in interactive sessions.

documentation : Human-language text written to explain what software does, how it works, or how to use it.

dotted notation : A two-part notation used in many programming languages in which thing.component refers to the component belonging to thing.

empty string : A character string containing no characters, often thought of as the "zero" of text.

encapsulation : The practice of hiding something's implementation details so that the rest of a program can worry about what it does rather than how it does it.

floating-point number : A number containing a fractional part and an exponent. See also: integer.

for loop : A loop that is executed once for each value in some kind of set, list, or range. See also: while loop.

function call : A use of a function in another piece of software.

immutable : Unchangeable. The value of immutable data cannot be altered after it has been created. See also: mutable.

import : To load a library into a program.

in-place operators : An operator such as += that provides a shorthand notation for the common case in which the variable being assigned to is also an operand on the right hand side of the assignment. For example, the statement x += 3 means the same thing as x = x + 3.

index : A subscript that specifies the location of a single value in a collection, such as a single pixel in an image.

inner loop : A loop that is inside another loop. See also: outer loop.

integer : A whole number, such as -12343. See also: floating-point number.

invariant : An expression whose value doesn't change during the execution of a program, typically used in an assertion. See also: precondition, postcondition.

library : A family of code units (functions, classes, variables) that implement a set of related tasks.

loop variable : The variable that keeps track of the progress of the loop.

member : A variable contained within an object.

method : A function which is tied to a particular object. Each of an object's methods typically implements one of the things it can do, or one of the questions it can answer.

object : A collection of conceptually related variables (members) and functions using those variables (methods).

outer loop : A loop that contains another loop. See also: inner loop.

parameter : A variable named in the function's declaration that is used to hold a value passed into the call. The term is often used interchangeably (and inconsistently) with argument.

pipe : A connection from the output of one program to the input of another. When two or more programs are connected in this way, they are called a "pipeline".

postcondition : A condition that a function (or other block of code) guarantees is true once it has finished running. Postconditions are often represented using assertions.

precondition : A condition that must be true in order for a function (or other block of code) to run correctly.

regression : To re-introduce a bug that was once fixed.

return statement : A statement that causes a function to stop executing and return a value to its caller immediately.

RGB : An additive model that represents colors as combinations of red, green, and blue. Each color's value is typically in the range 0..255 (i.e., a one-byte integer).

sequence : A collection of information that is presented in a specific order. For example, in Python, a string is a sequence of characters, while a list is a sequence of any variable.

shape : An array's dimensions, represented as a vector. For example, a 5×3 array's shape is (5,3).

silent failure : Failing without producing any warning messages. Silent failures are hard to detect and debug.

slice : A regular subsequence of a larger sequence, such as the first five elements or every second element.

stack frame : A data structure that provides storage for a function's local variables. Each time a function is called, a new stack frame is created and put on the top of the call stack. When the function returns, the stack frame is discarded.

standard input : A process's default input stream. In interactive command-line applications, it is typically connected to the keyboard; in a pipe, it receives data from the standard output of the preceding process.

standard output : A process's default output stream. In interactive command-line applications, data sent to standard output is displayed on the screen; in a pipe, it is passed to the standard input of the next process.

string : Short for "character string", a sequence of zero or more characters.

syntax error : A programming error that occurs when statements are in an order or contain characters not expected by the programming language.

test oracle : A program, device, data set, or human being against which the results of a test can be compared.

test-driven development : The practice of writing unit tests before writing the code they test.

traceback : The sequence of function calls that led to an error.

tuple : An immutable sequence of values.

type : The classification of something in a program (for example, the contents of a variable) as a kind of number (e.g. floating-point, integer), string, or something else.

type of error : Indicates the nature of an error in a program. For example, in Python, an IOError to problems with file input/output. See also: syntax error.

while loop : A loop that keeps executing as long as some condition is true. See also: for loop.