Python Library Quick Reference

This page describes the subset of the Python library that we have learned 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:

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.)
max(iter, [key = f])
max(arg1, arg2, …, [key = f])
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.
min(iter, [key = f])
min(arg1, arg2, …, [key = f])
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.
sorted(iter, [key = f])
Return a new sorted list of the items in iter. 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.

sequences

Sequences include lists, 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.
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.index(x)
Return the index of the first occurrence of x in s. If x is not in s, this raises a ValueError.
s.reverse()
Reverse the elements of a sequence in place.
Comparison operators such as ==, < and > also work on sequences. Operators such as < and > 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].
del l[i : j : k]
Delete a slice of elements from l[i] up to (but not including) l[j], stepping by k.
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.sort(key = None)
Sort a list in place. 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.isalnum()
True if all characters in s are letters or digits (and there is at least one character).
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.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.

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.

input and output

close(file)
Call this function 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.
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.
print(*objects, end = '\n', file = sys.stdout)
Print one or more objects. 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.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.deque()
Create a deque, i.e. a double-ended queue. deques support these operations:

import math

math.ceil(x)
Return x rounded up to the nearest integer.
math.cos(x)
Return the cosine of x in radians.
math.e
The constant e = 2.71828…
math.floor(x)
Return x rounded down to the nearest integer.
math.inf
A floating-point number representing positive infinity.
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.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.

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.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.

import random

This module generates pseudo-random numbers.

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.uniform(a, b)
Return a random float x in the range a ≤ x ≤ b.

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.

import time

time.time()
Return a float representing the number of seconds elapsed since January 1, 1970.