Convert

Python types can be converted in many ways. Often we employ built-in methods like str() or int or float. Often no custom code is needed.

More complex conversions often involve strings. We can convert number to strings, and strings to numbers—this requires parsing a string.

Tuple, list

This program creates a tuple with 3 elements, and then converts that tuple into a list with the list() method. This call returns a list.

vegetables = ("CARROT", "SQUASH", "ONION")

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

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

tup = ("RABBIT", "MOUSE", "BIRD", "GIRAFFE")

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

# Join tuple with slash.
s = "/".join(tup)
print(s)
RABBITMOUSEBIRDGIRAFFE
RABBIT/MOUSE/BIRD/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.

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)
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.

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)
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.

floating = 1.23456

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

# 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)
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.

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)
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.

value = "cat"

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

print(list)
['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.

# 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)
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.

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

def kilobytes_to_megabytes(kilobytes):
    return kilobytes / 1024

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

# 1024 kilobytes to megabytes.
megabytes2 = kilobytes_to_megabytes(1024)
print(1024, "kilobytes =", megabytes2, "megabytes")
100000 bytes = 0.095367431640625 megabytes
1024 kilobytes = 1.0 megabytes

Dict

With the dictionary built-in, we can convert from a list of tuples (with keys, values) to a dictionary. Dict() is a useful built-in method.

Summary

Some conversions in Python are numeric—these can often be done with mathematical expressions. For compound types such as collections, we use built-in methods to convert, such as list().