Python Library Quick Reference

This page describes the subset of the Python library that we will use in Programming 1. For more information, see the official Python Standard Library reference.

booleans

A bool is a Boolean value, either False or True. The function bool(x) converts a value to a bool.

Booleans provide the following operations:

numbers

An int is an integer of arbitrary size. A float is a floating-point number. The functions int(x) and float(x) convert a value to an int or float.

These numeric types provide these operations:

The following built-in functions work on numeric values:

abs(x)
Return the absolute value of x.
round(x)
Return x rounded to the nearest integer.

floats provide this operation:

f.is_integer()
Return true if a float represents an integer.

iterables

An iterable is a series of values that the for statement can loop over. All sequences (see below) are iterables, and Python supports various other kinds of iterables as well (e.g. sets, dictionaries, text files).

The following operations work on any iterable:

x in iter
True if x is a member of the given iterable. (For strings, however, s in t returns True if s is a substring of t.)
all(iterable)
Return True if all elements of an iterable are True.
any(iterable)
Return True if any elements of an iterable are True.
enumerate(iterable)
Return an iterable of pairs (i, x), where i is the index of an item in the given iterable and x is its value.
filter(function, iterable)
Apply a function to each element of an iterable, returning an iterable of all values for which the function returns True.
map(function, iterable)
Apply a function to each element of an iterable, returning an iterable of the results.
max(iter, *, key = None)
max(arg1, arg2, …, key = None)
Return the maximum value in an iterable, or in a series of arguments. If a key function is specified, it is applied to each value before computing the maximum. If the iterable is empy, the function raises a ValueError.
min(iter, *, key = None)
min(arg1, arg2, …, key = None)
Return the minimum value in an iterable, or in a series of arguments. If a key function is specified, it is applied to each value before computing the maximum. If the iterable is empy, the function raises a ValueError.
sorted(iter, *, reverse = False, key = None)
Return a new sorted list of the items in iter. If 'reverse' is True, the list will be sorted in reverse order. If a key function is specified, it is applied to each item to produce a comparison key.
sum(iter)
Return the sum of values in an iterable.
zip(*iterables)
Iterate over two or more iterables in parallel, returning an iterable of tuples. For example, zip(a, b) will produce an iterable of pairs, each containing an item from a and the corresponding item from b. zip() stops as soon as it reaches the end of the shortest of the given iterables.

sequences

Sequences include lists, tuples, ranges, and strings. Their elements can also be accessed directly by index. Sequences are iterable, i.e. the 'for' statement can loop over a sequence's elements.

Sequences support the following operations:

s[i]
Return the i-th element of s. The first element is s[0]. A negative value of i indexes from the end of the sequence: s[-1] is the last element, and s[-2] is the second last.
s[i : j]
Return a slice of elements from s[i] up to (but not including) s[j]. Either i or j may be negative to index from the end of the sequence. If i is omitted, it is 0. If j is omitted, it is len(s).
s[i : j : k]
Return a slice of elements from s[i] up to (but not including) s[j], stepping by k.
len(s)
Return the length of a sequence.
reversed(s)
Return an iterator over a sequence's elements in reversed order.
s + t
Concatenate two sequences of the same type. (This operator does not work on ranges.)
n * s (or s * n)
Concatenate s to itself n times. (This operator does not work on ranges.)
s.count(x)
Return the number of occurrences of x in s.
s.index(x)
Return the index of the first occurrence of x in s. If x is not in s, this raises a ValueError.
Comparison operators such as < and > work on some sequences, such as lists, tuples, and strings. These operators compare sequences lexicographically.

lists

A list is a mutable sequence of arbitrary values. The function list() converts any sequence to a list.

Lists implement all of the common sequence operations above, plus these additional operations:

del l[i]
Delete the element at index i.
del l[i : j]
Delete a slice of elements from l[i] up to (but not including) l[j].
l.append(x)
Append an element to a list.
l.clear()
Remove all elements from a list.
l.copy()
Return a shallow copy of a list.
l.extend(seq) (or l += seq)
Append a sequence of elements to a list.
l.insert(i, x)
Insert x into the list at index i. If i = 0, x is inserted at the beginning. If i = len(x), x is inserted at the end.
l.pop(i = -1)
Delete l[i] from the list and return it. If i is not provided, it defaults to -1, which removes and returns the element at the end of the list.
l.remove(x)
Remove the first list element that is equal to x. If there is no such element, raises a ValueError exception.
l.reverse()
Reverse the elements of a list in place.
l.sort(*, reverse = False, key = None)
Sort a list in place. If 'reverse' is True, the list will be sorted in reverse order. If a key function is specified, it is applied to each item to produce a comparison key.

ranges

A range is an immutable sequence of numbers.

range(stop)
range(start, stop, [step])
Return a range of integers from start up to (but not including) stop. If start is absent, it is 0. If step is absent, it is 1.

strings

A string is an immutable sequence of Unicode characters. The function str(x) converts a value to a string.

Strings implement the common sequence operations above, plus these operations:

chr(i)
Convert a Unicode code point (an integer) into a string containing the corresponding character. The inverse of this function is ord().
ord(c)
Given a string containing a Unicode character, return the character's code point, which is an integer. The inverse of this function is chr().
s.endswith(t)
True if s ends with the string t.
s.find(sub, [start], [end])
Return the lowest index in the string where substring sub is found within the slice s[start:end], or -1 if not found. start and end default to 0 and len(s).
s.isalpha()
True if all characters in s are alphabetic (and there is at least one character).
s.isdigit()
True if all characters in s are digits (and there is at least one character).
s.islower()
True if all letters in s are lowercase (and there is at least one letter).
s.isspace()
True if all characters in s are whitespace (and there is at least one character).
s.isupper()
True if all letters in s are uppercase (and there is at least one letter).
s.join(seq)
Join the strings in the given sequence into a single string, inserting s between each adjacent pair in the output.
s.lower()
Return a copy of a string with all characters converted to lowercase.
s.replace(old, new)
Return a copy of s with all instances of old replaced by new.
s.rstrip()
Remove whitespace from the end of a string.
s.split(sep = None)
Return a list of words in a string. By default, strings are separated by arbitrary sequences of whitespace characters. If sep is given, it is a delimiter string that separates words.
s.startswith(t)
True if s starts with the string t.
s.strip()
Remove whitespace from the beginning and end of a string.
s.upper()
Return a copy of the string with all characters converted to uppercase.

The string module defines these constants:

string.ascii_lowercase
The lowercase letters 'abcdefghijklmnopqrstuvwxyz'.
string.ascii_uppercase:
The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.

string formatting

A formatted string (f-string) such as f'the sum is {x + y}' can contain interpolated values surrounded in braces. Each interpolated value may be followed by a colon and a format code. Common format codes include

The 'd' and 'f' format codes may be preceded by ',' to indicate that digits should be grouped, producing a string such as '223,566,221'.

Any format code may be preceded by an integer indicating a field width, which is a minimum number of characters to output. If the interpolated value is shorter than the field width, Python will insert spaces at the beginning. For example, f'{23:5d}' produces '   23' (with three leading spaces). If a field width is preceded by a 0, then Python inserts zeroes rather than spaces. For example, f'{23:05d}' produces '00023'.

sets

A set is a mutable unordered collection of values. The set() function converts a sequence to a set.

Sets provide these operations:

len(s)
Return the number of elements in s.
x in s
Test for membership in a set.
s.add(x)
Add an element to a set.
s.clear()
Remove all elements from a set.
s.discard(x)
Remove an element from a set if it is present.
s.remove(x)
Remove an element from a set. An error will occur if the element is not present.
s & t
Return the intersection of two sets.
s | t
Return the union of two sets.
s - t
Return the difference of two sets.
s <= t
s < t
Return true if s is a subset (<=) or proper subset (<) of t.
s >= t
s > t
Return true if s is a superset (>=) or proper superset (>) of t.

dictionaries

A dictionary is a mapping from keys to values. The function dict(s) converts a sequence to a dictionary.

Dictionaries support these operations:

len(d)
Return the number of key/value pairs in a dictionary.
d[key]
Look up a key, returning the associated value. Raises an error if the key is not present.
d[key] = value
Set d[key] to a value.
del d[key]
Delete a key from a dictionary.
key in d
Tests to see whether a key is present in a dictionary.
d.clear()
Remove all items from a dictionary.
d.copy()
Return a shallow copy of a dictionary.
d.get(key, [default])
Look up a key and return the associated value, or the specified default value if the key is absent. If no default is specified, it is None.
d.items()
Return a sequence of key-value pairs in a dictionary.
d.keys()
Return a sequence of keys in a dictionary.
d.values()
Return a sequence of values in a dictionary.

objects

These built-in functions work on any objects:

isinstance(x, class)
Return True if the object x belongs to the given class.
type(x)
Return an object representing the type (i.e. class) of the object x.

input and output

input([prompt])
Read a line of standard input and return it as a string. If a prompt string is provided, it is printed before reading. The returned string will not include a newline character at the end. If the end of input is reached, this function raises an EOFError.
open(filename, mode = 'r')
Open a file and return a file object. mode contains one or more characters indicating how the file is to be accessed. The most important of these are
This method will raise a FileNotFoundError if the file is not found. FileNotFoundError has an attribute 'filename' containing the name of the file that was not found.
print(*objects, sep = ' ', end = '\n', file = sys.stdout)
Print one or more objects, separated by sep. By default, print() writes a newline character after each object; to change this behavior, pass a different value to the end parameter. print() writes to standard output by default, but you may pass a file object to the file parameter to specify a different destination.
The following are methods on file objects:
f.close()
Call this method to close a file object when you have finished reading from or writing to it. Closing after writing is especially important to ensure that all output will actually be written to the file.
f.read(size = -1)
Read at most size characters from a file and return them as a string. If size is -1 (the default value), read the entire file all at once.
f.readline()
Read a line from a text file. The returned string will end with a newline character ('\n'). If there are no more lines available, the method returns an empty string ('').
f.write(s)
Write a string to a text file. This method does not automatically append a newline to the output.

Also, any file object opened for reading in text mode is an iterable of strings, so you can loop over it with the for statement. Each string in the iteration will end with a newline character, just as if you had read the string with the readline() method.

import collections

collections.defaultdict(f)
Create and return a dictionary with default values. If code attempts to retrieve a missing key from the dictionary, the key will be added automatically and will be given a value returned by calling the given function f. The built-in functions int and list (which return 0 and [], respectively) are useful values for f.
collections.deque([iter])
Create a deque, i.e. a double-ended queue. If an iterable is provided as an argument, the deque will be initialized with elements from it.
deques support these operations:

import csv

csv.DictReader(file, *, delimiter = ',')
Create an iterable object that will read from the given CSV (comma-separated values) file, returning each row as a Python dictionary. file should be a file object returned by open(). The first line of the file is assumed to contain column names.
You may specify a delimiter character that separates fields in each line; the default delimiter is a comma.
csv.DictWriter(file, fieldnames, *, delimiter = ',')
Create an object that can write rows to a CSV file. file should be a file object that was opened for writing. fieldnames is a list of column names.
You may specify a delimiter character that separates fields in each line; the default delimiter is a comma.
dw.writeheader()
Write the header line to a CSV file. Typically you will call this method once after creating a DictWriter.
dw.writerow(d)
Given a dictionary containing a set of fields, write a line containing its data to a CSV file.

import datetime

date class

The date class represents a date such as January 7, 2024.

date(year, month, day)
Create a date object with the given year, month, and day.
date.today()
Create a date object representing today's date.
d.day
d.month
d.year
Retrieve the day, month, or year of a date object.
d.isoformat()
Convert a date to a string such as '2024-01-07'.

time class

The time class represents a time such as 13:18:02.

time(hour, minute, second)
Create a time object with the given hour, minute, and second.
t.hour
t.minute
t.second
Return the hour, minute, or second of a time object.

datetime class

The datetime class represents a date and time, such as January 7, 2024 at 13:18:02.

datetime(year, month, day, hour, minute, second)
Create a datetime object with the given attributes.
datetime.now()
Create a datetime object represent the current date and time.
dt.year
dt.month
dt.day
dt.hour
dt.minute
dt.second
Retrieve the year, month, day, hour, minute, or second of a datetime object.
dt.isoformat()
Convert a datetime to a string in a format such as '2024-01-07T13:18:02'.

import math

math.acos(x)
Return the arccosine of x in radians, a value between 0 and π.
math.asin(x)
Return the arcsine of x in radians, a value between -π/2 and π/2.
math.atan2(y, x)
Return the arctangent of (y / x) in radians, a value between -π and π.
math.ceil(x)
Return x rounded up to the nearest integer.
math.comb(n, k)
Return n choose k, i.e. the number of ways to choose k items from n items without repetition and without order.
math.copysign(x, y)
Return a float that has the magnitude of x but the sign (positive or negative) of y. (If y is 0, the result will be positive.)
math.cos(x)
Return the cosine of x in radians.
math.dist(p, q)
Return the Euclidean distance between points p and q, which may be tuples with any number of coordinates.
math.e
The constant e = 2.71828…
math.factorial(n)
Return n!.
math.floor(x)
Return x rounded down to the nearest integer.
math.inf
A floating-point number representing positive infinity.
math.isqrt(n)
Retrn the integer square root of n, i.e. the square root of n rounded down to the nearest integer.
math.log(x [, base])
Return the logarithm of x to the given base. The base defaults to e if it is not provided.
math.pi
The constant π = 3.14159…
math.prod(iter)
Return the product of the numbers in a list or other iterable.
math.sin(x)
Return the sine of x in radians.
math.sqrt(x)
Return the square root of x.
math.tan(x)
Return the tangent of x in radians.
math.tau
The constant τ = 2π = 6.283185…

import os

In the methods below, path is a string representing a filename, possibly with one or more leading directory components. For example, path could be
os.listdir(path)
Return a list of all filenames in the directory given by path.
os.makedirs(path, *, exist_ok = False)
Create a directory (including any parent directories as necessary). If exist_ok is True, no error is raised if the directory already exists.
os.remove(path)
Delete a file.
os.path.basename(path)
Return the base name of the given path. For example, the base name of '/foo/bar/xyz.txt' is 'xyz.txt'.
os.path.dirname(path)
Return the directory name of the given path. For example, the directory name of '/foo/bar/xyz.txt' is '/foo/bar'.
os.path.exists(path)
Return true if path is an existing file or directory.
os.path.isdir(path)
Return true if path exists and is a directory.
os.path.isfile(path)
Return true if path exists and is a regular file.
os.path.join(*paths)
Join one or more paths, inserting directory separator characters ('/' or '\') between path components.
os.system(command)
Execute a shell command. You can use this function to run an external program.

import random

This module generates pseudo-random numbers.

random.choice(seq)
Return a random element from a non-empty sequence.
random.randint(a, b)
Return a random integer n in the range a ≤ n ≤ b.
random.random()
Return a random float x in the range 0.0 ≤ x < 1.0.
random.randrange([a], b)
Return a random integer n in the range a ≤ n < b. a defaults to 0.
random.seed(a)
Seed the random number generator with an integer value. If you use the same seed in two runs of your program, you'll get the same random number sequence.
random.uniform(a, b)
Return a random float x in the range a ≤ x ≤ b.

import re

This module implements regular expressions. Here is a summary of regular expression syntax.

functions

re.findall(pattern, string)
Return a list of strings representing all non-overlapping matches of pattern in string, in left-to-right order. If pattern contains two or more groups, each list element is actually a tuple of strings.
re.finditer(pattern, string)
Return an iterable of match objects representing all matches of pattern in string.
re.fullmatch(pattern, string)
If the entire string matches the regular expression pattern, return a match object representing the match; otherwise return None.
re.match(pattern, string)
If the beginning of the string matches the regular expression pattern, return a match object representing the match; otherwise return None.
re.search(pattern, string)
If the string contains a match for the regular expression pattern, return a match object representing the first such match. If there is no match, return None.
re.sub(pattern, replacement, string)
Return a string generated by replacing all occurrences of pattern in string with replacement.

match objects

A match object represents a match of a regular expression at a particular position in a string. In a boolean context, a match object is always True.

m[number]
If number is 0, return the entire text that matched. If number > 0, return the string matching the corresponding parenthesized group.
m.groups()
Return a tuple containing the strings that matched each group in the pattern.

import sys

sys.argv
A program's command-line arguments. sys.argv[0] is the name of the program itself, and the following elements are the actual arguments.
sys.exit()
Exit the program immediately.
sys.stdin
The program's standard input. This is an iterable object representing a series of lines of text. Each line will end with a newline character ('\n').

import time

time.sleep(secs)
Wait for the given number of seconds, which may be a floating-point number.
time.time()
Return a float representing the number of seconds elapsed since January 1, 1970.

import matplotlib.pyplot as plt

Matplotlib is a library for visualizing data. To install it, run

$ pip install matplotlib
plt.bar(xs, ys)
Plot data as a series of vertical bars.
plt.barh(xs, ys)
Plot data as a series of horizontal bars.
plt.figure(*, figsize = (x, y))
Create a new figure (i.e. a graph). x and y specify its width and height in inches.
plt.grid(*, axis = 'both')
Enable grid lines on a plot. The axis parameter indicates the axis on which to draw grid lines, and may be 'x', 'y', or 'both'.
plt.legend()
Display a legend, which shows labels for each plot on the graph.
plt.plot(xs, ys, symbol = '-', *, label = string)
Plot data. xs contains a series of numeric x-values, and ys contains a corresponding series of y-values. 'symbol' is a marker for plotting points, such as 'o', 'x', or '+'; if omitted, a line plot is drawn. You may specify a label for the plot.
plt.savefig(filename)
Save the current plot to the given file. The filename's extension determines the format in which to save. For example, 'my_plot.svg' will be saved in SVG format.
plt.show()
Display the current graph.
plt.title(string)
Set a title for the graph.
plt.xlabel(string)
Set a label for the x-axis.
plt.xlim(lo, hi)
Specify low and high values for the x-axis.
plt.ylabel(string)
Set a label for the y-axis.
plt.ylim(lo, hi)
Specify low and high values for the y-axis.