Variables in Python are easy to learn but often have unexpected quirks. This makes typeerrors an especially common issue when exploring new areas of the language. But if you’ve found yourself confronted with a “typeerror: a bytes-like object is required, not ‘str'” error then you’ll be happy to know it’s generally fairly easy to fix.
What Does the Typerror Mean?
This error is fairly straightforward on the surface. It simply means that an incorrect datatype has been passed within Python. Specifically, a Python string has been passed instead of Python’s bytes-like object. But fixing the typeerror: a bytes-like object is required not str error requires delving a little deeper into the difference between a string and a bytes like object
The Difference Between a String and a Byte-Like Object
One of the reasons why this error is so common is that it’s easy to confuse a string and a bytes-like object. Consider the following code, which will ultimately fail with the “a bytes-like object is required, not ‘str'” error message.
The first and second line both create a variable containing “Hello Blorld”. However, the b when declaring varTwo tells the interpreter to create a bytes object rather than one of the standard strings which we might expect. The next line prints the contents of varOne. And we follow that by doing the same with varTwo.
The print command highlights two important points which explain why this typeerror typically occurs. Strings and bytes-like objects both print out the same way. They also appear almost identical to the human eye. You might easily just glance at a bytes-like object and not see anything amiss within the content of a large printout.
The reason for the similarity is that Python’s formatting creates Unicode strings when you pass plain text to a variable declaration. And bytes are simply basic data, which can in fact be a Unicode string. For example, you might load “Hello Blorld” from a text file on your computer. It would be quite similar to strings with the same text. But Python’s interpreter will treat it differently. This is because Python’s system looks at the content, but not necessarily context, of a binary file. It sees the bytes which create the Unicode string. But it doesn’t understand that it’s a Unicode string unless you specifically redefine it as such. When a bytes-like object is sent to a print statement, Python’s interpreter simply empties out what’s in memory without necessarily understanding what’s being presented.
The following two lines make a call to the type function, which shows that we are indeed looking at two very different things. The type call’s output shows that we’re actually working with strings and bytes-like objects rather than just strings. We then try to perform a replace to fix the “Blorld” typo in varThree. This process works as expected and we see the result printed to screen. Next, we try to do the same with varFour. This causes the script to crash with the familiar typeerror.
The reason for the crash is that we’re trying to use a replace consisting of Unicode sequences on a variable that consists of bytes. The error message is telling us that if we want to use a replace on a bytes-like object then we need to pass data formatted as a byte.
Note that in real-world situations the mismatch between the byte object and str object will seldom be as self-apparent within your code. The misidentification usually comes about when you’re reading unknown data into a variable. One of the most common examples comes from working with files. Python’s system has different ways of working with files, typically declared when accessing them for the first time. Consider these two lines.
The first line passes a file name and ‘rb’. While the second does the same but omits the ‘b’. Each line opens example.txt for reading. But the ‘rb’ also tells Python’s interpreter that we want to read the data as binary rather than as text mode strings. It’s easy to omit the proper declaration of read mode. And this is especially true when working with files which might be intentionally opened in binary format while others aren’t. It’s important to always keep careful track of the mode used to work with files as they can produce unpredictable data if too many assumptions are made when accessing them.
This error will also occur in functions that require byte-like objects to be passed during the initial call or with an instance method. The socket functions are some of the most common examples within Python’s standard library. For example, the send function of socket requires you to encode all strings as binary data before passing them.
Fixing the Typeerror
We can go about fixing this issue in a few different ways. But the simplest is to simply convert our strings to bytes-like objects. We can do so with the previous examples by just changing one line of code. Run the previous code again, but use this line again for the varFour replace.
The primary modification comes from passing the output of bytes to replace rather than directly supplying strings. Bytes, as the name suggests, convert the passed variable to bytes-like objects. The second variable consists of the binary mode encoding. Python’s system defaults to 8-bit instances of the Unicode transformation format (UTF-8) as the default encoding method. But for the sake of code clarity, it’s always a good idea to be explicit in binary mode encoding. This makes it easier to add a proper bytes class decode for the variables if needed.
You can also use string’s encode and decode instead of bytes. For example, you might have a string called myString. You could encode it to a bytes-like object with the following syntax.
As with bytes, encode will default to utf-8. But again, it’s usually best to go with a more explicit declaration of the format for the sake of clarity. One of the nice things about these methods is that they’re universally applicable to most situations where you need to convert a string to a bytes-like object. The typeerror can arise from a number of different problems. But it can typically be fixed by simply ensuring overall parity between two data types by using conversions.