Python Convert Types: Tuple, List and Dictionary

Convert. A tuple is not a list.

This is normally not a problem. But when we have another method that requires a list, we must convert that tuple.

To transform types, we have many options in Python. Often we employ built-in methods like str or int or float. But sometimes custom methods are needed.

Tuple, list. This program creates a tuple with three elements. It then converts that tuple into a list with the list() method. This call returns (naturally) a list.

Result: The program shows that the two collections are separate. They occupy different locations in memory.

List: The list built-in method copied all the tuple's elements into a separate, new collection.

Based on:

Python 3

Python program that converts tuple to list

vegetables = ("carrot", "squash", "onion")

# Convert to list.
veg2 = list(vegetables)
veg2.append("lettuce")

# Print results.
print(vegetables)
print(veg2)

Output

('carrot', 'squash', 'onion')
['carrot', 'squash', 'onion', 'lettuce']

Tuple, string. A tuple can be converted into a single string. This is best done with the string join method. Join() is called on a delimiter string.

Tip: We pass Join() a tuple, and the result is a string containing the tuple's items.

Python program that converts tuple, string

tup = ("rabbit", "mouse", "gorilla", "giraffe")

# Join tuple with no space.
s = "".join(tup)
print(s)

# Join tuple with slash.
s = "/".join(tup)
print(s)

Output

rabbitmousegorillagiraffe
rabbit/mouse/gorilla/giraffe

List, string. Lists can also be converted into strings with the join method. This code sample is the same as one that converts a tuple, but it uses a list.

Separators: We join() together the strings with no separator, and with a semicolon.

Tip: Join can handle any iterable collection, not just tuples and lists. Try it with a set.

Python program that converts list, string

vehicles = ["car", "truck", "tractor"]

# Convert list to string with join.
result = "".join(vehicles)

# Convert with semicolons separating strings.
result2 = ";".join(vehicles)

print(result)
print(result2)

Output

cartrucktractor
car;truck;tractor

Dictionary, list. With the list() method, we can also convert a dictionary to a list. There is some complexity here. When we call items() on a dictionary, we get a view.

And: A view is different from a list. It cannot be manipulated in the same ways.

Items: We get a view of the dictionary with items() and then convert that to a list. So we actually convert the dictionary to a list.

Tip: With dictionary views, we can change the order. For example, we can use the sorted() method to reorder elements in a view.

Python program that converts dictionary

vegetables = {"carrot": 1, "squash": 2, "onion": 4}

# Convert dictionary to list of tuples.
items = list(vegetables.items())

for item in items:
    print(len(item), item)

Output

2 ('carrot', 1)
2 ('squash', 2)
2 ('onion', 4)

Int built-in. Numbers can be converted to different types. In many languages this is called explicit casting. In Python we use a method syntax, such as int(), to do this.

Int: This method receives a number and returns the integral form of that number.

So: If we pass 1.5 to int(), it will return 1. It always removes the fractional part of the number.

Python program that converts to int

floating = 1.23456

# Convert to int and print.
print(floating)
print(int(floating))

Output

1.23456
1

Int, string. A number is converted into a string with str. And a string is converted into a number with int. In this example, we do these two conversions.

And: We show the value types by using them. We use len() on the string and add one to the number.

So: The string "123" has three characters. And the number 123 is increased to 124 when we add one to it.

Python program that converts int, string

# Convert number to string.
number = 123
value = str(number)

# Print the string and its character count.
print(value)
print(len(value))

# Convert string to number.
number2 = int(value)

# Print the number and add one.
print(number2)
print(number2 + 1)

Output

123
3
123
124

Class, string. We can specify how a class is converted to a string with the __repr__ method. This method returns a string. We can have it return values of fields in the class.

Note: The repr and str methods (with no underscores) are used on instances of a class. If the __repr__ method is defined, it will be used.

Tip: Classes like list also have repr (representation) methods. This is why you can directly print a list.

Python that converts class to string

class Test:
    def __init__(self):
        self.size = 1
        self.name = "Python"

    def __repr__(self):
        # Return a string.
        return "Size = " + str(self.size) + ", Name = " + str(self.name)

t = Test()

# Str and repr will both call into __repr__.
s = str(t)
r = repr(t)

# Display results.
print(s)
print(r)

Output

Size = 1, Name = Python
Size = 1, Name = Python

String, chars. We can get the chars from a string with a list comprehension. This syntax uses an inner loop expression to loop over each char in the string. This results in a list of chars.

Python that gets list of chars

value = "cat"

# Get chars from string with list comprehension.
list = [c for c in value]

print(list)

Output

['c', 'a', 't']

Bytes, string. Python 3 has the space-efficient bytes type. We can take a string and convert it into bytes with a built-in. We provide the "ascii" encoding as a string argument.

Decode: To convert from the bytes data back into a string, we can use the decode method. We again must provide an encoding string.

Python that converts bytes, string

# Convert from string to bytes.
value = "carrot"
data = bytes(value, "ascii")
print(data)

# Convert bytes into string with decode.
original = data.decode("ascii")
print(original)

Output

b'carrot'
carrot

Bytes, megabytes. A number in one unit, like bytes, can be converted into another, like megabytes. Here we divide by 1024 twice. Further conversions (bytes, gigabytes) are possible.

Note: Large files have many bytes. In an interface, displaying this number is awkward and hard to read.

Convert: One megabyte contains 1024 kilobytes. And 1 kilobyte contains 1024 bytes.

So: To get from bytes to megabytes, we divide by 1024, and then by 1024 again: two divisions are needed.

And: To go from kilobytes to megabytes, we need just one division by 1024. This is simple, but testing is important.

Python that converts bytes, megabytes

def bytestomegabytes(bytes):
    return (bytes / 1024) / 1024

def kilobytestomegabytes(kilobytes):
    return kilobytes / 1024

# Convert 100000 bytes to megabytes.
megabytes1 = bytestomegabytes(100000)
print(100000, "bytes =", megabytes1, "megabytes")

# 1024 kilobytes to megabytes.
megabytes2 = kilobytestomegabytes(1024)
print(1024, "kilobytes =", megabytes2, "megabytes")

Output

100000 bytes = 0.095367431640625 megabytes
1024 kilobytes = 1.0 megabytes

A summary. Conversions in Python use many syntax forms. Some conversions are numeric. These can be done with mathematical methods or arithmetic.

Often, conversions are encapsulated in a method definition. And for compound types such as collections, the Python language provides many built-in methods to convert, such as list().

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