Convert Types
This page was last reviewed on Dec 22, 2022.
Dot Net Perls
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.
String conversions. 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. It then converts that tuple into a list with the list() method. This call returns a list.
Result The program shows that the 2 collections are separate. They occupy different locations in memory.
Detail The list built-in method copied all the tuple's elements into a separate, new collection.
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.
Tip We pass Join() a tuple, and the result is a string containing the tuple's items.
tup = ("rabbit", "mouse", "gorilla", "giraffe") # Join tuple with no space. s = "".join(tup) print(s) # Join tuple with slash. s = "/".join(tup) print(s)
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.
Detail 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.
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.
And A view is different from a list. It cannot be manipulated in the same ways.
Detail 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.
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.
Detail 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.
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.
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.
# 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.
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.
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.
Detail To convert from the bytes data back into a string, we can use the decode method. We again must provide an encoding 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)
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.
Detail 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.
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")
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.
A conversion note. If your conversion requires processing, consider a custom method. In programming, conversions are often necessary. Reducing conversions usually improves performance.
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().
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
No updates found for this page.
© 2007-2024 Sam Allen.