Python Reference

Created by Nishant Kheterpal and Jessica Hu

Table Functions and Methods

In the examples in the left column, np refers to the NumPy module, as usual. Everything else is a function, a method, an example of an argument to a function or method, or an example of an object we might call the method on. For example, tbl refers to a table, array refers to an array, and num refers to a number. array.item(0) is an example call for the method item, and in that example, array is the name previously given to some array.

Name Chapter Description Input Output
Table() 6 Create an empty table, usually to extend with data None An empty Table
Table().read_table(filename) 6 Create a table from a data file string: the name of the file Table with the contents of the data file
tbl.with_columns(name, values) tbl.with_columns(n1, v1, n2, v2,...) 6 A table with an additional or replaced column or columns. name is a string for the name of a column, values is an array 1. string: the name of the new column;
2. array: the values in that column
Table: a copy of the original Table with the new columns added
tbl.column(column_name_or_index) 6 The values of a column (an array) string or int: the column name or index array: the values in that column
tbl.num_rows 6 Compute the number of rows in a table None int: the number of rows in the table
tbl.num_columns 6 Compute the number of columns in a table None int: the number of columns in the table
tbl.labels 6 Lists the column labels in a table None array: the names of each column (as strings) in the table
tbl.select(col1, col2, ...) 6 Create a copy of a table with only some of the columns. Each column is the column name or index. string or int: column name(s) or index(es) Table with the selected columns
tbl.drop(col1, col2, ...) 6 Create a copy of a table without some of the columns. Each column is the column name or index. string or int: column name(s) or index(es) Table without the selected columns
tbl.relabel(old_label, new_label) 6 Modifies the existing table in place, changing the column heading in the first argument to the second 1. string: the old column name
2. string: the new column name
Table: a copy of the original with the changed label
tbl.sort(column_name_or_index) 6.1 Create a copy of a table sorted by the values in a column. Defaults to ascending order unless descending = True is included. 1. string or int: column index or name
2. (Optional) descending = True
tbl.where(column, predicate) 6.2 Create a copy of a table with only the rows that match some predicate See Table.where predicates below. 1. string or int: column name or index
2. are.(...) predicate
tbl.take(row_indices) 6.2 A table with only the rows at the given indices. row_indices is an array of indices. array of ints: the indices of the rows to be included in the Table Table: a copy of the original with only the rows at the given indices
tbl.scatter(x_column, y_column) 7 Draws a scatter plot consisting of one point for each row of the table. Note that x_column and y_column must be strings specifying column names. 1. string: name of the column on the x-axis
2. string: name of the column on the y-axis
None: draws a scatter plot
tbl.plot(x_column, y_column) 7 Draw a line graph consisting of one point for each row of the table. 1. string: name of the column on the x-axis
2. string: name of the column on the y-axis
None: draws a line graph
tbl.barh(categories) tbl.barh(categories, values) 7.1 Displays a bar chart with bars for each category in a column, with height proportional to the corresponding frequency. values argument unnecessary if table has only a column of categories and a column of values. 1. string: name of the column with categories
2. (Optional) string: the name of the column with values for corresponding categories
None: draws a bar chart
tbl.hist(column, unit, bins) 7.2 Generates a histogram of the numerical values in a column. unit and bins are optional arguments, used to label the axes and group the values into intervals (bins), respectively. Bins have the form [a, b), where a is included in the bin and b is not. 1. string: name of the column with categories
2. (Optional) string: units of x-axis
3. (Optional) array of ints/floats denoting bin boundaries
None: draws a histogram
tbl.apply(function, column) 8.1 Returns an array of values resulting from applying a function to each item in a column. 1. function: function to apply to column
2. string: name of the column to apply function to
array: contains an element for each value in the original column after applying the function to it
tbl.group(column_or_columns, func) 8.2 Group rows by unique values or combinations of values in a column(s). Multiple columns must be entered in array or list form. Other values aggregated by count (default) or optional argument func. 1. string or array of strings: column(s) on which to group
2. (Optional) function: function to aggregate values in cells (defaults to count)
Table: a new Table
tbl.pivot(col1, col2, vals, collect) tbl.pivot(col1, col2) 8.3 A pivot table where each unique value in col1 has its own column and each unique value in ccol2 has its own row. Count or aggregate values from a third column, collect with some function. Default vals and collect return counts in cells. 1. string: name of column whose unique values will make up columns of pivot table
2. string: name of column whose unique values will make up rows of pivot table
3. (Optional) string: name of column that describes the values of cell
4. (Optional) function: how the values are collected, e.g. sum or np.mean
Table: a new Table
tblA.join(colA, tblB, colB) tblA.join(colA, tblB) 8.4 Generate a table with the columns of tblA and tblB, containing rows for all values of a column that appear in both tables. Default colB is colA. colA and colB must be strings specifying column names. 1. string: name of column in tblA with values to join on
2. Table: other Table
3. (Optional) string: if column names are different between Tables, the name of the shared column in tblB
Table: a new Table
tbl.sample(n) tbl.sample(n, with_replacement) 10 A new table where n rows are randomly sampled from the original table. Default is with replacement. For sampling without replacement, use argument with_replacement=False. For a non-uniform sample, provide a third argument weights=distribution where distribution is an array or list containing the probability of each row. 1. int: sample size
2. (Optional) with_replacement=True
Table: a new Table with n rows


Array Functions and Methods

Name Chapter Description
max(array) 3.3 Returns the maximum value of an array
min(array) 3.3 Returns the minimum value of an array
sum(array) 3.3 Returns the sum of the values in an array
abs(num), np.abs(array) 3.3 Take the absolute value of number or each number in an array.
round(num), np.round(array) 3.3 Round number or array of numbers to the nearest integer.
len(array) 3.3 Returns the length (number of elements) of an array
make_array(val1, val2, ...) 5 Makes a numpy array with the values passed in
np.average(array) np.mean(array) 5.1 Returns the mean value of an array
np.diff(array) 5.1 Returns a new array of size len(arr)-1 with elements equal to the difference between adjacent elements; val_2 - val_1, val_3 - val_2, etc.
np.sqrt(array) 5.1 Returns an array with the square root of each element
np.arange(start, stop, step) np.arange(start, stop) np.arange(stop) 5.2 An array of numbers starting with start, going up in increments of step, and going up to but excluding stop. When start and/or step are left out, default values are used in their place. Default step is 1; default start is 0.
array.item(index) 5.3 Returns the i-th item in an array (remember Python indices start at 0!)
np.random.choice(array, n) np.random.choice(array) 9 Picks one (by default) or some number 'n' of items from an array at random. By default, with replacement.
np.count_nonzero(array) 9 Returns the number of non-zero (or True) elements in an array.
np.append(array, item) 9.2 Returns a copy of the input array with item (must be the same type as the other entries in the array) appended to the end.
percentile(percentile, array) 12.1 Returns the corresponding percentile of an array.


Table.where Predicates

Any of these predicates can be negated by adding not_ in front of them, e.g. are.not_equal_to(Z) or are.not_containing(S).

Predicate Description
are.equal_to(Z) Equal to Z
are.above(x) Greater than x
are.above_or_equal_to(x) Greater than or equal to x
are.below(x) Less than x
are.below_or_equal_to(x) Less than or equal to x
are.between(x,y) Greater than or equal to x and less than y
are.between_or_equal_to(x,y) Greater than or equal to x, and less than or equal to y
are.contained_in(A) Is a substring of A (if A is a string) or an element of A (if A is a list/array)
are.containing(S) Contains the string S
are.strictly_between(x,y) Greater than x and less than y


Miscellaneous Functions

These are functions in the datascience library that are used in the course that don't fall into any of the categories above.

Name Chapter Description Input Output
sample_proportions(sample_size, model_proportions) 11.1 Sample_size should be an integer, model_proportions an array of probabilities that sum up to 1. The function samples sample_size objects from the distribution specified by model_proportions. It returns an array with the same size as model_proportions. Each item in the array corresponds to the proportion of times it was sampled out of the sample_size times. 1. int: sample size
2. array: an array of proportions that should sum to 1
array: each item corresponds to the proportion of times that corresponding item was sampled from model_proportions in sample_size draws, should sum to 1
minimize(function) 15.4 Returns an array of values such that if each value in the array was passed into function as arguments, it would minimize the output value of function. function: name of a function that will be minimized. array: An array in which each element corresponds to an argument that minimizes the output of the function. Values in the array are listed based on the order they are passed into the function; the first element in the array is also going to be the first value passed into the function.