I want to walk through a practical example. A very common use case for working with the files system is to log values. What we're going to do is we're going to build a small program here that is going to be able to create a logger. We're gonna see how we can create a file, and then add it to it.

Then we're also going to walk through a few potential issues that you may run into. Let's start off. I have a file here called file builder, and I'm going to create a variable here where I'm going to open up a file. I'm going to show you here in the console that if I type ls, you can see we do not have a file called logger.

That is what we are going to be creating. Let's create this variable. It's going to be file_builder, and then I'm going to call the function open. open is a function built into the Python core language, and it is going to allow us to open or create a file.

Now one thing that I want to point out is that the function name here might be a little bit confusing. The word open kind of makes you assume that it is opening up a preexisting file; however, the way that Python works is if you call open, if it finds a file then it will open up that file.

Then you can perform whatever you need to inside of it. If it does not find a file with that name, then it will automatically create it. So I'm going to call open here and pass in the name of the file. So I can say:

file_builder = open("logger.txt", 

This is just going to be a regular text file, and it takes another argument which is the way that we want to open it up. This is the set of permissions that we want our program to follow. It takes in a string and I'm gonna say:

file_builder = open("logger.txt", "w+")

This allows us to write to this file, and then we close off the parens. That is going to then open up that file. Now the very first thing I want to do is just show you how to add content dynamically into the file. I can say file_builder.write. So write is a function inside of the file library in Python and then inside of it I'm just going to say:

file_builder = open("logger.txt", "w+")

file_builder.write("Hey, I'm in a file!")

Let's just make sure that this is working. Now after you have written a file, and you've called that write function, you need to close the file off. So I'm going to say file_builder.close() and then we're going to call the close function.

file_builder = open("logger.txt", "w+")

file_builder.write("Hey, I'm in a file!")

file_builder.close()

It's not going to take any arguments. It simply is going to say this file has been opened, it was written, and now we're closing it off. Nothing thing else is going to be added to that. If I run this code, I'm going to say python file_guide.py, and hit return. We don't get any errors.

Now if you type ls, you can see that we have a logger file. Let's open it up, and you can see that our text "Hey, I'm in a file" has been added to that. That is working nicely.

Now let's take something that's a little bit more practical. I'm going to just comment this out for us so that you can have it in the show notes. Now let's build an actual logging system. This is going to be a little bit more practical for how you typically use this. So I'm going to say:

file_builder = open("logger.txt", "w+")

for i in range(100):

# file_builder.write("Hey, I'm in a file!")

file_builder.close()

Then inside of this for-in-loop, what I want to do is I want to call the same exact process. I can say:

file_builder = open("logger.txt", "w+")

for i in range(100):
    file_builder.write()


# file_builder.write("Hey, I'm in a file!")

file_builder.close()

In a formatted string, I'm going to say "I am on line", and then using the string literal interpolation I'll say {i + 1}. The reason why I'm adding one is because I want to increment it, and I do not want it to start at 0. When you called range it would start at zero, so I actually want to just start on one.

So you are going to say I'm on line and then it's going to give that dynamic line number. Then I want to add a carriage return. The way you can do that is right after the curly braces pass in \n, and then close off the formatted string, close off the file builder right function, and hit save.

file_builder = open("logger.txt", "w+")

for i in range(100):
    file_builder.write(f"I'm on line {i + 1}\n")


# file_builder.write("Hey, I'm in a file!")

file_builder.close()

Now if I come over here, and I run that same command of python file_guide.py, it runs right away. Let's take a look at that logger now. Now you can see that it went and built that entire program.

If you come all the way down to the bottom. You can see it as I'm on line 100. On every one of these lines, it gives the line number actually lists it out. That is how you can have your dynamic numbers. Now, one thing you may have noticed where originally we said Hey, I'm in a file, that has been overwritten.

Whenever you use this kind of syntax where you are opening a file and then you're just calling right on it, it is not going to care whatsoever about the contents of the file previously.

This is what you would do if you want to have almost like a temp type of logger. You only want to have these files available and the data inside them available in a temporary fashion.

In the next guide, we're going to walk through how we can append to a file. Say that we wanted to come here and instead of 100, we want to change this to 1000. Then we can run this again.

file_builder = open("logger.txt", "w+")

for i in range(1000):
    file_builder.write(f"I'm on line {i + 1}\n")


# file_builder.write("Hey, I'm in a file!")

file_builder.close()

You can see how fast it is. It works very quickly. Now if we take a look, you can see even though it has these numbers, it'll have 1 to 100 and that looks the same, these are actually written completely from scratch.

One of the elements I want you to truly remember from this guide is that if you use this syntax, where you're opening a file and you're simply calling right you will overwrite all of the content in that file. If I go all the way to the bottom you can see it goes all the way down to line number 1000.

So in review, that is the basic introduction for how you can create and then add content to a file in Python.

Code

file_builder = open("logger.txt", "w+")

for i in range(1000):
    file_builder.write(f"I'm on line {i + 1}\n")


# file_builder.write("Hey, I'm in a file!")

file_builder.close()