Save as binary file python

But before you can parse data, you need to load the data. We will use the simplest approach as described above. There are faster ways, but it is usually better to get a working program first before you try to make it go fast call "optimizing".

We will put our controlling code into a function called "main". Functions in a module aka python file can come in any order, but I put my main functions at the bottom of the file. This is just my personal style. Here is the program with the new main function that uses the open and read calls discussed previously. It still doesn't do anything useful, but we are getting closer!

Give it a try to make sure it works. You can see the prints at the beginning and ending of the main function. Now we need to pass in the data that we loaded in main to the decode function. We must add an argument to the decode function.

The name of this argument has nothing to do with the name of what we pass in. What ever is passed in first will get assigned to the first argument inside the function. So here, the contents of the sbet data variable will get assigned to the "data" variable inside the decode function. It is also time to start doing a bit of documentation.

Python has a mechanism called "doc strings". If there is a string as the first line of a file, function, class, or class method, then python considers that string as the documentation for the class.

In the above example, I have added a string right after the definition of the decode function. There are two ways to ask for help from ipython: the help command and appending a?

So we have documentation, but a function that doesn't do anything. I know you are getting impatient with me, so let's actually decode the first datagram in the SBET file. Since the decode function uses unpack from the struct library and degrees from the math library, we need to add an import statement to the top of the file.

You might wonder why if we have done an import of math in ipython, why do we need it in the file? Each module is independent and needs to tell python which modules it needs. This is pretty exciting! We now have a file that we can use to redo a decoding. It's not very flexible, but it works. We need to work to improve it to be more useful. The first thing to do to make it more useful is to allow it to run from the command line.

We can almost do that already, but not as easily as we would like. First quit ipython and get back to the bash terminal prompt. We can tell python to run a file:. We need to add some special code that detects if the python is being run as a main program. Add this line to your sbet. Run ipython and import sbet. Using this, we can add a check at the bottom of sbet. If so, we can right away start up the process of getting the script going.

If sbet. Then if we run the script using python from the bash prompt, it will actually call the main function and get things going. It can be really annoying to have to remember to type python before the script all the time. You don't want to be required to tell other people which language the script uses any time they want to run it. There is a special mechanism that bash uses to see how it should run a script file: it checks the first line. If that line starts with a!

The trouble is that we don't know exactly where python is installed on the computer. Plus the person might have changed their path to point to a different version of python that works better for them. We want to honor that choice. There is a unix command called env that is supposed to always be located in the same place.

It looks at the path and finds the python interpreter. This is the recommended way to create a python script. Add this line to the very beginning of your sbet. One more thing to fix.

You have to tell the computer that the file is "executable". Otherwise it will not think that it can run the file. The chmod command will let you set the permissions on the sbet. After chmod is run, not the extra "x" characters for executable on the left side of the return from the ls command.

Before we go any farther, let's clean up the file and give it a look over. I have deleted extra print statements. Now let's change the decode function to get all of the fields as we did before. I am going to use the dict and zip method before, but if you are writing your own decoder for something else, you can certainly decode each field individually. Add the field names to your file before the decode function and change decode to look like this:. I have change the printing of the result at the end to be easier to read.

If we just print a dictionary, it will be a big mush. However, looping over a dictionary with a for loop will return the keys of the dictionary one by one. Try running the resulting program. Really, when programming, it is a bad idea to mix the logic and data handling with the "view" or output of the program.

What if you want to parse a datagram, but have no need to print it? We should split the printing part into a separate program, but that means our decode function needs to return back what it figured out so we can pass it on.

To get the datagram information back from the decode function. The way to do that is to "return" the dictionary sbet values back at the end of the decode and to remove the printing. Then we need to take that printing code from before and make a function for it.

I would use "print" as the function name, but that is already taken by python's print, so I will call it sbet print. We haven't done anything to how the script looks when it is run, but now we have a program that is a little bit more flexible. However, we now have the problem that we can only decode the first datagram.

It would be nice to be able to decode any datagram from the file that we would like. First, we need to modify the decode function to know where the datagram starts in the data.

We can pass in a second argument to decode that tells it how far into the data we want it to look for the datagram. This is commonly referred to as the offset. We can also tell python that if decode is called without an offset, it is okay to start at the beginning of the file. This is done by setting offset to the default value of 0. Now we need to know how many datagrams are in the file and where each datagram starts. Change Language.

Related Articles. Table of Contents. Improve Article. Save Article. Like Article. Code to demonstrate bitwise operations. Shifting right will lose the. Shifting left will add a 0 bit. Attention geek! Strengthen your foundations with the Python Programming Foundation Course and learn the basics. Perkins: Sure, and you should avoid generator expressions if you need to work on 2.

But 2. If you need to support 2. It doesn't matter what type your bytes come from. I'm not sure bytearray is a good choice for file writing. You would need to limit the size to manageable chunks.

Otherwise once your filesizes get too high you will run out of memory. Mark Ransom Mark Ransom k 40 40 gold badges silver badges bronze badges. However, you are correct I think in this case, since the example data includes non printable characters.

Even if they are,. You're right, struct. MarkRansom: Well, this is still definitely a good solution to the more general problem of "I have a list of integers of some arbitrary but fixed size, how can I write them to a binary file? Show 7 more comments. You can also shorten the last two lines into a single one: newFile. CrepeGoat CrepeGoat 1, 16 16 silver badges 20 20 bronze badges.

Perkins Perkins 2, 21 21 silver badges 20 20 bronze badges. The ascii encoding only works in python2. You can use the following code example using Python 3 syntax: from struct import pack with open "foo. How to load and save 3D Numpy array to file using savetxt and loadtxt functions? Python - Combine two dictionaries having key of the first dictionary and value of the second dictionary.

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