Programming with Python
- Import a library into a program using
- Use the
numpy library to work with arrays in Python.
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.
print(something) to display the value of
- The expression
array.shape gives the shape of an array.
array[x, y] to select a single element from an array.
- Array indices start at 0, not 1.
low:high to specify a slice that includes the indices from
- All the indexing and slicing that works on arrays also works on strings.
# some kind of explanation to add comments to programs.
numpy.min(array) to calculate simple statistics.
numpy.mean(array, axis=0) or
numpy.mean(array, axis=1) to calculate statistics across the specified axis.
- Use the
pyplot library from
matplotlib for creating simple visualizations.
for variable in collection to process the elements of a collection one at a time.
- The body of a for loop must be indented.
len(thing) to determine the length of something that contains other values.
[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).
glob.glob(pattern) to create a list of files whose names match a pattern.
* in a pattern to match zero or more characters, and
? to match any single character.
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.
== 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.
- Define a function using
- The body of a function must be indented.
- Call a function using
- 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.
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).
- 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
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 strings will generate errors if you try to access items in them that do not exist. This type of error is called an
- 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
- 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.
- 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.
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.
- additive color model
- A way to represent colors as the sum of contributions from primary colors such as red, green, and blue.
- A value given to a function or program when it runs. The term is often used interchangeably (and inconsistently) with parameter.
- 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.
- To give a value a name by associating a variable with it.
- (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.
- Treating text as if upper and lower case characters of the same letter were the same. See also: case-sensitive.
- Treating text as if upper and lower case characters of the same letter are different. See also: case-insensitive.
- 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.
- To apply one function to the result of another, such as
- 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.
- A character or characters used to separate individual values, such as the commas between columns in a CSV file.
- 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.
- 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
- empty string
- A character string containing no characters, often thought of as the “zero” of text.
- 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.
- A group of instructions (i.e., lines of code) that transform some input arguments to some output.
- function call
- A use of a function in another piece of software.
- Unchangeable. The value of immutable data cannot be altered after it has been created. See also: mutable.
- 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.
- 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.
- A whole number, such as -12343. See also: floating-point number.
- An expression whose value doesn’t change during the execution of a program, typically used in an assertion. See also: precondition, postcondition.
- 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.
- A variable contained within an object.
- 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.
- 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.
- 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.
- 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”.
- A condition that a function (or other block of code) guarantees is true once it has finished running. Postconditions are often represented using assertions.
- A condition that must be true in order for a function (or other block of code) to run correctly.
- 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.
- 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).
- 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.
- An array’s dimensions, represented as a vector. For example, a 5×3 array’s shape is
- silent failure
- Failing without producing any warning messages. Silent failures are hard to detect and debug.
- 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.
- 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.
- The sequence of function calls that led to an error.
- An immutable sequence of values.
- 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.