An introduction to Python: Glossary

Python is just another program on your computer.

Python can be used interactively through a Read-Eval-Print-Loop.

Using Python interactively is great to manipulate data and exploratory work.

Use variables to store values.

Use print to display values.

Variables persist between cells.

Variables must be created before they are used.

Variables can be used in calculations.

Use an index to get a single character from a string.

Use a slice to get a substring.

Use the built-in function len to find the length of a string.

Python is case-sensitive.

Use meaningful variable names.

Every value has a type.

Use the built-in function type to find the type of a value.

Types control what operations can be done on values.

Strings can be added and multiplied.

Strings have a length (but numbers don’t).

Must convert numbers to strings or vice versa when operating on them.

Can mix integers and floats freely in operations.

Variables only change value when something is assigned to them.

Python scripts are plain text files.

Spyder is a software that integrates both a text editor and the Python runtime.

Spyder provides convenience features such as autocompletion, documentation lookup and debugging.

Spyder is one of many options.

Use comments to add documentation to programs.

A function may take zero or more arguments.

Commonly-used built-in functions include max, min, and round.

Functions may only work for certain (combinations of) arguments.

Functions may have default values for some arguments.

Use the built-in function help to get help for a function.

Every function returns something.

Python reports a syntax error when it can’t understand the source of a program.

Python reports a runtime error when something goes wrong while a program is executing.

Fix syntax errors by reading the source code, and runtime errors by tracing the program’s execution.

Most of the power of a programming language is in its libraries.

A program must import a library module in order to use it.

Use help to learn about the contents of a library module.

Import specific items from a library to shorten programs.

Create an alias for a library when importing it to shorten programs.

Import a library into a program using import libraryname.

Use the numpy library to work with arrays in Python.

The expression array.shape gives the shape of an array.

Use array[x, y] to select a single element from a 2D array.

Array indices start at 0, not 1.

Use low:high to specify a slice that includes the indices from low to high-1.

Use # some kind of explanation to add comments to programs.

Use numpy.mean(array), numpy.max(array), and numpy.min(array) to calculate simple statistics.

Use numpy.mean(array, axis=0) or numpy.mean(array, axis=1) to calculate statistics across the specified axis.

Use the pyplot module from the matplotlib library for creating simple visualizations.

Use the Pandas library to get basic statistics out of tabular data.

Use index_col to specify that a column’s values should be used as row headings.

Use DataFrame.info to find out more about a dataframe.

The DataFrame.columns variable stores information about the dataframe’s columns.

Use DataFrame.T to transpose a dataframe.

Use DataFrame.describe to get summary statistics about data.

Use DataFrame.iloc[..., ...] to select values by integer location.

Use : on its own to mean all columns or all rows.

Select multiple columns or rows using DataFrame.loc and a named slice.

Result of slicing can be used in further operations.

Use comparisons to select data based on value.

Select values or NaN using a Boolean mask.

[value1, value2, value3, ...] creates a list.

Lists can contain any Python object, including lists (i.e., list of lists).

Lists are indexed and sliced with square brackets (e.g., list[0] and list[2:9]), 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).

A for loop executes commands once for each value in a collection.

A for loop is made up of a collection, a loop variable, and a body.

The first line of the for loop must end with a colon, and the body must be indented.

Indentation is always meaningful in Python.

Loop variables can be called anything (but it is strongly advised to have a meaningful name to the looping variable).

The body of a loop can contain many statements.

Use range to iterate over a sequence of numbers.

The Accumulator pattern turns many values into one.

Use if statements to control whether or not a block of code is executed.

Conditionals are often used inside loops.

Use else to execute a block of code when an if condition is not true.

Use elif to specify additional tests.

Conditions are tested once, in order.

Create a table showing variables’ values to trace a program’s execution.

Use a for loop to process files given a list of their names.

Use glob.glob to find sets of files whose names match a pattern.

Use glob and for to process batches of files.

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 when trying to use a variable that does not exist. Possible causes are that a variable definition is missing, a variable reference differs from its definition in spelling or capitalization, or the code contains a string that is missing quotes around it.

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 IndexError.

Trying to read a file that does not exist will give you an FileNotFoundError. Trying to read a file that is open for writing, or writing to a file that is open for reading, will give you an IOError.

Break programs down into functions to make them easier to understand.

Define a function using def with a name, parameters, and a block of code.

Defining a function does not run it.

Arguments in call are matched to parameters in definition.

Functions may return a result to their caller using return.

The scope of a variable is the part of a program that can ‘see’ that variable.