CS50 Library for Python

Installation

$ pip3 install cs50

Usage

Note

To use these functions, be sure to include import cs50 atop your file.

cs50.get_float(prompt)
Parameters

prompt – the str with which to prompt the user for input

Returns

the float equivalent to the line read from stdin as precisely as possible, or None on error

Prompts user for a line of text from standard input and returns the equivalent float; if text does not represent a floating-point value or would cause overflow or underflow, user is reprompted.

Example usage:

f = get_float("Input a floating-point number: ")
cs50.get_int(prompt)
Parameters

prompt – the str with which to prompt the user for input

Returns

the int equivalent to the line read from stdin, or None on error

Prompts user for a line of text from standard input and returns the equivalent int; if text does not represent an integer, user is reprompted.

Example usage:

f = get_int("Input an integer: ")
cs50.get_string(prompt)
Parameters

prompt – the str with which to prompt the user for input

Returns

the read line as a string sans line endings, or None on EOF.

Prompts user for a line of text from standard input and returns it as a str, sans trailing line ending. Supports CR (\r), LF (\n), and CRLF (\r\n) as line endings.

Example usage:

s = get_string("Input a string: ")
cs50.SQL(url)
Parameters

url – a str that indicates database dialect and connection arguments

Returns

a cs50.SQL object that represents a connection to a database

Example usage:

db = cs50.SQL("sqlite:///file.db")  # For SQLite, file.db must exist
db = cs50.SQL("mysql://username:password@host:port/database")  # For MySQL
db = cs50.SQL("postgres://username:password@host:port/database")  # For PostgreSQL
cs50.SQL.execute(sql, *args, **kwargs)
Parameters

  • sql – a str that represents a single SQL statement, possibly with parameter markers, with or without a trailing semicolon

  • *args – zero or more positional arguments with which any parameter markers should be substituted

  • **kwargs – zero or more named arguments with which any parameter markers should be substituted

Any argument whose value is a list or tuple of other values is converted to a comma-separated list of those values, formatted for SQL’s IN operator. Any argument whose value is None is converted to NULL for SQL.

Supported placeholders include question marks (qmark style) and named placeholders (named style).

Returns

  • for CREATE, True on success or False on failure

  • for DELETE, the number of rows deleted

  • for INSERT, the primary key of a newly inserted row (or None if none)

  • for SELECT, a list of dict objects, each of which represents a row in the result set

  • for UPDATE, the number of rows updated

  • on integrity errors (as when a constraint is violated), a ValueError is raised

  • on other errors, a RuntimeError is raised

Example usage:

import cs50

db = cs50.SQL("sqlite:///file.db")

rows = db.execute("SELECT * FROM foo")

rows = db.execute("SELECT * FROM foo WHERE bar = ? AND baz = ?", 1, 2)
rows = db.execute("SELECT * FROM foo WHERE bar IN (?) AND baz IN (?)", [1, 2], [3, 4])

rows = db.execute("SELECT * FROM foo WHERE bar = :bar AND baz = :baz", bar=1, baz=2)
rows = db.execute("SELECT * FROM foo WHERE bar IN (:bar) AND baz IN (:baz)", bar=[1, 2], baz=[3, 4])

id = db.execute("INSERT INTO foo (bar, baz) VALUES(?, ?)", 1, 2)
id = db.execute("INSERT INTO foo (bar, baz) VALUES(:bar, :baz)", bar=1, baz=2)

n = db.execute("UPDATE foo SET bar = ?, baz = ?", 1, 2)
n = db.execute("UPDATE foo SET bar = :bar, baz = :baz", bar=1, baz=2)

n = db.execute("DELETE FROM foo WHERE bar = ? AND baz = ?", 1, 2)
n = db.execute("DELETE FROM foo WHERE bar = :bar AND baz = :baz", bar=1, baz=2)

FAQs

How can I use % with LIKE?

If s is a str, you can prepend and/or append % to it as follows:

rows = db.execute("SELECT * FROM foo WHERE bar LIKE ?", "%" + s + "%")

How can I add optional clauses to a query?

Based on user input, you might want to include or not include some clauses in a query. For instance, you might want to include bar and baz in a query only if they have values, in which case the number of placeholders you have in your query might vary. You could thus construct your query dynamically by joining the clauses and unpacking the placeholders’ values as follows:

query = "SELECT * FROM foo"

clauses = []
values = []

if bar:
    clauses.append("bar = ?")
    values.append(bar)
if baz:
    clauses.append("baz = ?")
    values.append(baz)

if clauses:
    query = query + " WHERE " + " AND ".join(clauses)
rows = db.execute(query, *values)

The end result is equivalent to:

rows = db.execute("SELECT * FROM foo WHERE bar = ? AND baz = ?", bar, baz)

But you don’t need to know in advance if bar and baz will have values.

How come I can’t use parameter markers as placeholders for tables’ or columns’ names?

Parameter markers (e.g., ?) can only be used as placeholders for “literals” like integers and strings, not for “identifiers” like tables’ and columns’ names. If a user’s input will determine the table or column on which you execute a statement, you can use a format string (f-string) instead, but you must validate the user’s input first, to ensure the table or column exists, lest you risk a SQL-injection attack, as in the below:

if column in ["foo", "bar", "baz"]:
    rows = db.execute(f"SELECT * FROM {column}")

How can I enable logging of SQL statements?

By default, logging of SQL statements is disabled unless you have an environment variable called FLASK_ENV, the value of which is development, as is the case in Visual Studio Code for CS50. You can enable logging of SQL statements with code like:

import logging

logging.getLogger("cs50").disabled = False

Statements that succeed will be prefixed with INFO. Statements that fail will be prefixed with ERROR.

How can I disable logging of SQL statements?

To disable logging of SQL statements, even if you have an environment variable called FLASK_ENV, the value of which is development, as is the case in Visual Studio Code for CS50, you can use code like:

import logging

logging.getLogger("cs50").setLevel("CRITICAL")

Troubleshooting

ModuleNotFoundError: No module named ‘_sqlite3’

If on an apt-based system, try:

apt install libsqlite3-dev

If on a yum-based system, try:

yum install sqlite-devel