glossary

## Python Glossary

#### argument
A value given to a function or program when it runs. The term is often used interchangeably (and inconsistently) with [parameter](#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](#invariant), [precondition](#precondition), [postcondition](#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.

#### boolean operators
logical operators, i.e. `and`, `or`, and `not`, that allow us to build more complex conditional statements from simpler conditional statements. See: [conditional statements](#conditional-statement)

#### 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).

#### case-sensitive
Treating text as if upper and lower case characters of the same letter are different. See also: [case-insensitive](#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. Also called "boolean expression."

#### 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](#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](#comma-separated-values) 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](#integer).

#### for loop
A loop that is executed once for each value in some kind of set, list, or range. See also: [while loop](#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](#mutable).

#### import
To load a [library](#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](#outer-loop).

#### integer
A whole number, such as -12343. See also: [floating-point number](#floating-point-number).

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

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

#### mutable
Changeable. The value of mutable data can be altered after it has been created. See also: [immutable](#immutable).

#### object
A collection of conceptually related variables ([members](#member)) and functions using those variables ([methods](#method)).

#### outer loop
A loop that contains another loop. See also: [inner loop](#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](#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](#assertion).

#### 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.

#### sequence
A collection of information that is presented in a specific order. For example, in Python, a [string](#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](#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](#pipe), it receives data from the [standard output](#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](#pipe), it is passed to the [standard input](#standard-input) of the next process.

#### string
Short for "character string", a [sequence](#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](#immutable) [sequence](#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](#float), [integer](#integer)), [string](#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](#syntax-error).

#### while loop
A loop that keeps executing as long as some condition is true. See also: [for loop](#for-loop).

#### whitespace
Any character or series of characters that represent horizontal or vertical space. Generally, space-bar, tab, return/enter. Some whitespace in python is represented differently than what you see. A return is `\n` and tab `\t`.