For and While Loops in Python 3

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Programs often have to run the same commands over and over again. Python provides two types of loop statements to handle two different situations. The Python for loop is used when the number of iterations is known before the loop starts running. In contrast, the Python while loop repeats as long as a certain condition is true. This tutorial describes how to use both types of loops and explains how to use Python for common scenarios like looping through a dictionary.

An Introduction to Python for and while Loops

The Python for statement is a control flow statement that executes a specific block of code a certain number of times. Without this command, programming would be tedious and much less useful. Each for loop contains a sequencer that determines how many times the loop should run. Some loops use the range function to delimit the starting and stopping points. Other loops use a sequential data structure, such as a list, string, or dictionary, to define the sequence. In this case, the length of the data structure determines the range. The loop repeats once for each item in the structure.

A for loop is used whenever the loop should run a certain number of times. Under normal circumstances, changes inside the loop do not cause the loop to terminate early. However, the break statement allows for early termination of the loop under unexpected or adverse conditions. Here are some cases when a loop might be useful.

  • Count how many times each letter grade appears in a student transcript.
  • Given a list of employees, determine how many did not complete the mandatory ethics course.
  • Update a macro-economic model for each of the next ten years.

In these cases, the program can determine the range of the loop before it begins looping. In the first case, a finite list of grades is used as a sequencer for the loop. If the list is ['A', 'B', 'C', 'D', 'F'], then the loop iterates five times, once for each grade.

The Python while statement continues to execute a block of code as long as a test condition is true. The loop stops running when the condition no longer holds. Therefore, it is impossible to tell in advance how many times the loop might run. To determine whether the loop should iterate again, Python calculates the Boolean value of the statement’s conditional expression. An example of this type of expression is num_attempts < max_attempts. If the expression evaluates to True, then Python executes the loop again. If it is False, then Python stops running the loop. Control flow passes to the next statement after the loop.

As long as its expression continues to be True, a while loop can keep running indefinitely. For the loop to terminate, the conditional expression must change to False at some point. This means at least one of the variables used in the expression must be updated somewhere within the code block. A conditional expression that can never change to False leads to an infinite loop. This type of loop is defined as an endlessly repeating sequence of instructions. There are a few cases where this is desirable, for instance, in the kernel of an operating system. But most of the time, this is a bug. Always ensure the conditional expression can change given the correct exit conditions.

In certain situations, it is sensible to add a guard condition to a conditional expression. For example, a loop that validates a user password should limit the number of attempts. The guard often takes the form of a loop counter that increments with every iteration.

Here are a few examples of cases where a while statement could be used:

  • A user is repeatedly prompted for a password until they enter it correctly.
  • A program accepts and processes new input until the user enters an escape sequence.
  • A function continues reading data from a stream as long as the connection remains open.

To summarize, a for statement is used when the maximum number of iterations is known in advance. Use a while statement when the loop should keep running until something changes.

Before You Begin

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Note
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Python Loops

The Python for Loop

The Python for statement is a compound statement. It consists of a header and a block of code. The first line of the statement, up until the : symbol, is the header. The header contains the following components:

  1. The for keyword, which begins the statement.
  2. A loop variable, which is also known as the iterator. This variable is incremented or decremented with each new iteration of the loop. It is not necessary to define the loop variable beforehand. Python creates this variable when it is first used.
  3. The keyword in.
  4. A sequence to constrain how many times the loop executes. This is supplied using either the range function or a sequential data object. The built-in range function accepts up to three integers. These stand for the starting position, the ending position, and the step for the sequence. The sequential data type can be a String, List, Set, Tuple, or Dictionary. More details are provided in the following sections.
  5. A : symbol that terminates the statement header.

Each for statement is paired with a block of executable code, known as the suite. The code block consists of one or more indented lines of code. The first non-indented line of code terminates the code block. According to Python’s PEP 8 style guidelines, the indentation should be four spaces long.

At the beginning of each loop, Python increments the value of the iterator and verifies whether the loop should continue. If so, Python executes the code block again.

Using the Python for Loop with a Range

The Python for statement is frequently used with the range keyword. When the range function is used to calculate the sequence, the loop keeps running as long as the iterator falls within the range. When used with the range function, the syntax for a Python for loop follows the format below:

for iterator in range(start, end, step):
    statements

The range operator accepts up to three integer parameters:

  • start: This serves as the initial value of the iterator and the starting point of the range. If the start value is omitted, it defaults to 0. It is inclusive, which means the iterator is set to this value for the first instance of the loop.
  • end: This mandatory value determines the end position of the range. A comparison is made between the value of the iterator and the endpoint at the start of the loop. If the iterator is still within the range, the loop continues to iterate. This value is exclusive, so if end and the iterator are equal, the loop stops running.
  • step: The step indicates how much the iterator should increment each cycle. It is optional, and has a default value of 1. If a step is used, all three values must be specified.

To illustrate how these values work together, range(0,5) increments the iterator from 0 to 4 and runs five times. range(1,5,2) sets the iterator to 1 to start off and increments it by 2 with each iteration. It only runs two times, because the third time the iterator is 5, which is equal to the end value.

The following program demonstrates how to use the range function to constrain a Python for loop. The range uses the default start value and an end point of 5. The iterator i is set to zero at the start of the loop and continues to increment each cycle. The loop continues to run while i is less than 5. The code block prints out the new value of the iterator each time it runs.

File: loop1.py
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for i in range(5):
    print("The value of i is", i)
print("The loop has ended.")
python3 loop1.py
The value of i is 0
The value of i is 1
The value of i is 2
The value of i is 3
The value of i is 4
The loop has ended.

In the next example, the loop initializes i to 1 and adds 2 to i each loop, up to an upper limit of 7. When i is 5, the loop executes again. When it increments to 7, it is no longer less than the end value, so the loop terminates. Control passes to the next statement outside the loop.

File: loop2.py
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for i in range(1,7,2):
    print("The value of i is", i)
print("The loop has ended.")
python3 loop2.py
The value of i is 1
The value of i is 3
The value of i is 5
The loop has ended.

Negative values can be used for start or end points and for the step. When the step is negative, Python verifies whether the iterator is still greater than the end value. The following example decrements the iterator by 10 each time the loop begins.

File: loop3.py
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for i in range(5,-30,-10):
    print("The value of i is", i)
print("The loop has ended.")
python3 loop3.py
The value of i is 5
The value of i is -5
The value of i is -15
The value of i is -25
The loop has ended.

A for loop can also have an else statement, although it is not used too often. The code associated with the else statement runs after the loop ends. It can be used for post-loop processing or for situations when the loop does not run at all.

File: loop_else.py
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for i in range(5):
    print("The value of i is", i)
else:
    print("The for condition is false.")
print("The loop has ended.")
python3 loop_else.py
The value of i is 0
The value of i is 1
The value of i is 2
The value of i is 3
The value of i is 4
The for condition is false.
The loop has ended.
Note
The iterator continues to execute when the loop terminates. But it is not considered good programming practice to use it outside the loop. This behavior might vary between different versions and releases of Python.

Using the Python for Loop with Sequential Data Types

The Python for loop can also be used with sequential data structures such as Strings, Lists, Tuples, and Dictionaries. In these cases, the length of the data structure defines the range of the sequence. The loop continues to iterate while there are more items. For example, if a for loop uses a list containing five items, the loop iterates five times. Each item in the data structure is assigned in turn to the iterator.

When a for loop is used with a sequential data type, the syntax changes slightly. The range function is no longer used, but the structure is much the same otherwise.

for iterator in sequence:
    statements

How to Loop Through a List in Python

Python uses the List’s built-in __iter__ function to step through the List. Each new loop assigns the next item in the List to the iterator variable. The for loop is guaranteed to iterate through all items in the List in sequential order. It continues to iterate as long as the List contains more items.

The following example demonstrates how to use the Python for statement to loop through a List. In this case, the program defines a List named cities. The line for city in cities iterates through the cities List. At the start of each loop, it assigns the next item in the List to city. Upon each new loop, city contains the name of the next city. Within the code block, each city is printed on a new line.

File: loop_list.py
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cities = ['Chicago', 'Detroit', 'New York', 'Miami']
for city in cities:
    print("The next city is", city)
print("The loop has ended.")
python3 loop_list.py
The next city is Chicago
The next city is Detroit
The next city is New York
The next city is Miami
The loop has ended.

How to Loop Through a Dictionary in Python

Python can loop through a dictionary in much the same way it loops through a List. However, the structure of a dictionary is more complicated. A dictionary maps keys to values. Each dictionary item is a key-value pair which uses the key as its index. To be valid, a key must be comparable to other keys and cannot change. However, a value can be of any Python type, and types can be mixed within the same dictionary. A value can also be a sequential data type, such as a List or Tuple, or even another Dictionary.

Beginning with release 3.6, Python iterates over dictionary keys in the same order the entries were created. The for loop provides the name of the next key, not the value. However, the value can be accessed using the indexing operator []. To use the indexing operator, specify the name of the dictionary and append the indexing operator, placing the name of the key inside. The syntax for this operation is dictionary_name[key]. For example, if a dictionary named citystates contains a key named Miami, the associated value of Miami is retrieved using citystates[Miami].

Note
Earlier releases of Python often order dictionary entries differently.

The lopp_dcit.py file demonstrates how to loop through a Dictionary named citystates. In this Dictionary, the key is the name of the city while the value is the name of the state. More specifically, city contains the name of the city, and citystates[city] contains the name of the corresponding state. The code block prints both variables. Because the Dictionary contains four entries, the for loop iterates four times.

File: loop_dict.py
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citystates = {'Chicago' : 'Illinois', 'Detroit' : 'Michigan', 'New York' : 'New York', 'Miami' : 'Florida'}
for city in citystates:
    print("The name of the city is", city, "and the name of the state is", citystates[city])
print("The loop has ended.")
python3 loop_dict.py
The name of the city is Chicago and the name of the state is Illinois
The name of the city is Detroit and the name of the state is Michigan
The name of the city is New York and the name of the state is New York
The name of the city is Miami and the name of the state is Florida
The loop has ended.

Every Python Dictionary has a built-in method named .values. To use the .values method, append an empty arguments list (). This function supplies a view object containing all the values from the Dictionary without the corresponding keys. This method is useful when a program does not require the keys, only the Dictionary values. Although a view object is not actually a List, a for loop can process it in the same way. The for loop iterates through the view, supplying the next value in the view each time the loop runs.

Note
A view object is dynamic because it changes when the underlying object changes. However, it is possible to cast a view object into a non-dynamic List.

The following example loops through the Dictionary values. The loop retrieves each subsequent item in citystates.values(). It then prints the value, which is the name of the state. The key is never retrieved or used, and the indexing operator is not required. In this case, the loop iterates through a view object of the Dictionary values, not the Dictionary itself.

File: loop2_dict.py
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citystates = {'Chicago' : 'Illinois', 'Detroit' : 'Michigan', 'New York' : 'New York', 'Miami' : 'Florida'}
for state in citystates.values():
    print("The name of the state is", state)
print("The loop has ended.")
python3 loop2_dict.py
The name of the state is Illinois
The name of the state is Michigan
The name of the state is New York
The name of the state is Florida
The loop has ended.
Note
There is also a corresponding .keys() method that generates a view object containing the Dictionary’s keys. Using this view is almost the same as using the default method for iterating through a Dictionary. The Dictionary values are still accessed using the indexing operator.

The built-in items method generates a view object containing all the key-value pairs from a Python dictionary. Each pair is a tuple object. The values in a tuple can be separated using a technique known as tuple unpacking. To unpack a tuple, assign the tuple to a sequence of variables, separated by commas. The number of variables must match the number of items in the tuple. To assign the values of the two-item tuple myTuple to the variables x and y, use the statement x, y = myTuple. This assigns the first value in myTuple to x and the second value to y.

Note
A tuple is an immutable collection of objects separated by commas. To learn more about Python tuples, see the Python data structures documentation.

A for loop can be paired with the .items method to iterate through all key-value pairs in a Python dictionary. Upon each iteration through the view object, the next key-value tuple is extracted and unpacked to the loop variables. The following example assigns the key and value from citystates to the city and state variables. When this method is used, the indexing operator is no longer required to access the value.

File: loop3_dict.py
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citystates = {'Chicago' : 'Illinois', 'Detroit' : 'Michigan', 'New York' : 'New York', 'Miami' : 'Florida'}
for city, state in citystates.items():
    print("The name of the city is", city, "and the name of the state is", state)
print("The loop has ended.")
python3 loop3_dict.py
The name of the city is Chicago and the name of the state is Illinois
The name of the city is Detroit and the name of the state is Michigan
The name of the city is New York and the name of the state is New York
The name of the city is Miami and the name of the state is Florida

Built-in Python dictionary methods, like items() can be used to efficiently loop over a Python dictionary. To learn about these built-in dictionary methods, see our guide How to Use Dictionaries in Python 3.

How to Break or Exit from a For Loop in Python

The Python break statement immediately breaks out of the innermost for loop. It is often used to handle unexpected conditions or errors. For example, a loop might read data from a file and write it to a database. If the database suddenly becomes inaccessible, a break statement can immediately end the loop. Otherwise, the program would repeatedly keep timing out. It is better to give prompt feedback that something is not working.

In the loop_break.py example file, the break statement terminates the loop when the factorial of the iterator exceeds 5000. Based on the start and end values, the program is expected to loop through the code block five times. However, it stops after three cycles because the factorial of 7 is greater than the guard value of 5000.

Note
This program imports the built-in Python math module and uses the module’s factorial function.
File: loop_break.py
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import math
for i in range(5,10):
    print("The factorial of", i, "is", math.factorial(i))
    if math.factorial(i) > 5000:
        print("The limit of 5000 is exceeded.")
        break
print("The loop has ended.")
python3 loop_break.py
The factorial of 5 is 120
The factorial of 6 is 720
The factorial of 7 is 5040
The limit of 5000 is exceeded.
The loop has ended.

The Python while Loop

The Python while loop is similar to the for loop, but it is used in slightly different circumstances. It works best in situations where the number of iterations is not known when the program first begins to loop. A good example is a guessing game. The game ends when the player guesses the correct number. So the loop keeps running until the correct number is chosen.

Like the Python for loop, the Python while statement is structured as a compound statement. It also contains a header and an associated code block. This code block is executed every time the loop iterates. The entire code block must be indented, so the first non-indented line signifies the end of the block. The indentation should be four spaces.

The header for a while statement is somewhat simpler than the for loop header. It contains the following items:

  1. The while keyword.
  2. A conditional expression, which must evaluate to a Boolean value of either True or False. If the expression is True, Python executes the corresponding code block.
  3. A : symbol, terminating the header and introducing the code block.

A while loop does not have an iterator or a range and does not assign any variables. It does not lend itself to handling a sequential data structure. It can be thought of as an “indefinite” if statement. Upon each iteration, Python evaluates the Boolean expression. If it is True, the code block is executed. If it is False, the while loop terminates and control flow passes to the first non-indented line following its code block. The conditional expression can use any of Python’s logical or comparison operators, including ==, <=, or and. If the expression is False the first time through the while loop, the loop never runs at all.

As long as the Boolean expression remains True, the while statement keeps looping. To avoid an infinite loop, one of two events must happen. Either the conditional expression must eventually evaluate as False, or a break statement must be used inside the code block. For the expression to change, the code block must change one of the values used in the expression.

Note
To forcibly break out of an infinite loop in a Python program, enter Ctrl-C. This only works when running in interactive mode or running a Python program from the command line.

The syntax for a while loop is structured as follows:

while (Boolean_expression):
    statements

In this example, the while statement is used to validate a password. The program defines the password and then queries the user for their password. The user input is assigned to guess. If password != guess the loop continues to run. If the user does not remember their password, they can get stuck in an infinite loop. Therefore, it is good practice to add a guard counter to a while loop.

The following while statement keeps looping while the password is wrong and the user has made fewer than ten attempts to guess it. Inside the loop, the program increments the counter and prompts the user to guess the password. If the guess equals the password, the program tells the user they are right. Program control flow passes back to the start of the loop to see if the block should run again. However, this time password != guess is False and the loop terminates.

File: while.py
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password = 'linode'
guess = ''
counter = 0
while (password != guess) and (counter < 5):
    counter += 1
    print("Please enter your password: Attempt", counter)
    guess = input()
    if guess == password:
        print("The password is correct on attempt", counter)
python3 while.py
Please enter your password: Attempt 1
yes
Please enter your password: Attempt 2
password
Please enter your password: Attempt 3
linode
The password is correct on attempt 3

If the user never guesses the password in five attempts, the program ends gracefully.

Please enter your password: Attempt 1
a
Please enter your password: Attempt 2
b
Please enter your password: Attempt 3
c
Please enter your password: Attempt 4
d
Please enter your password: Attempt 5
e

How to Break or Exit from a While Loop in Python

The break statement is used more frequently inside a while loop than in a for loop. A break statement could be used when a special character or a control sequence can terminate a loop. Instead of setting a flag and then determining if the flag is set in the while conditional, it is easier to exit the loop immediately.

Python also supports a while else loop. This structure works the same way the for else statement does. The else clause is executed if the while statement conditional is False. This is used for post-processing or clean-up tasks when the precondition is reached. For example, it can be used for a good-bye message when the user enters a certain sequence. It is also commonly used when a positive response breaks the loop. In this case, the else statement only executes when the attempt fails.

The use of a break statement and an else directive make the preceding program more efficient. The program already compares password and guess when deciding whether to display a success message. Therefore, a break statement can be added right after the message, causing the loop to immediately terminate. The unnecessary comparison at the start of the next loop does not have to be performed.

Additionally, the conditional expression can be simplified. It only has to compare counter to the guard value. The program does not have to compare guess to password because it knows the two values are different. If the user had guessed correctly, making guess and password the same, the break statement would have been used. Therefore, password != guess is already known to be True and does not have to be re-evaluated. The only time when the conditional fails are when the maximum number of guesses has been made. This implies the user never entered the password, so the else clause can confidently display an error message.

File: while_break.py
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password = 'linode'
guess = ''
counter = 0
while counter < 5:
    counter += 1
    print("Please enter your password: Attempt", counter)
    guess = input()
    if guess == password:
        print("The password is correct on attempt", counter)
        break
else:
    print("The password was not entered correctly.")
python3 while_break.py
Please enter your password: Attempt 1
a
Please enter your password: Attempt 2
linode
The password is correct on attempt 2

If the user fails to guess correctly, the error statement is printed.

Please enter your password: Attempt 1
a
Please enter your password: Attempt 2
b
Please enter your password: Attempt 3
c
Please enter your password: Attempt 4
d
Please enter your password: Attempt 5
e
The password was not entered correctly.

The “do while” Loop in Python

Unlike the while loop, a do while loop evaluates the conditional expression at the end of the loop. Therefore it always executes the code block at least once. If the expression evaluates to True, the loop runs again.

Python does not support the do while loop structure even though it is available in many other languages. However, the equivalent logic can be implemented in Python using the statement while True: along with one or more break statements. The expression True is always True so the loop is guaranteed to run once. In fact, it runs until a break statement is encountered within the loop. A if statement inside the loop is used to determine when to break out of the loop. The code block must contain a break statement to terminate the loop. Otherwise, an infinite loop is formed.

Here is a basic outline demonstrating how a do while loop might be implemented in Python. This example breaks out of the loop on the tenth cycle.

File: do_while.py
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guard_limit = 10
i = 0
while True:
    i += 1
    print(i)
    if i == guard_limit:
        print("The break condition has been reached.")
        break
python3 do_while.py
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The break condition has been reached.

Summarizing the Python for and while Loops

Two Python statements are used to create loops. The Python for statement iterates for a fixed number of times. In contrast, a while statement keeps running as long as a conditional expression is satisfied. Both statements supply a block of code, which runs each time the loop iterates.

The for statement is often used with a range indicator that specifies the starting and ending points of the loop sequence. The range determines how many times the loop iterates. However, a for loop is also used to iterate through sequential data structures, including Lists and Dictionaries. At the start of each loop, either the next item in the structure or the next value in the sequence is assigned to the iterator.

A conditional expression constrains the Python while statement. If the expression evaluates to True, Python runs the corresponding code block. If it is False, the loop terminates. Unless the expression can change based on updates within the code block, it is possible to get trapped in an infinite loop.

Both the for and while statements can be paired with an else directive that only executes when the loop terminates. In addition, the Python break statement in the code block can be used to forcibly break out of the loop. This technique is handy for dealing with unexpected error conditions. For more information about the Python for and while statements, consult the Python documentation.

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