In this tutorial we are going to learn about Python File Operations such as python read file, python write file, open file, delete file and copy file.
Our previous tutorial was on Python Dictionary.
You can find that in this link. In the previous tutorial we used console to take input. Now, we will be taking input using file. That means, we will read from and write into files. To do so, we need to maintain some steps. Those are. Suppose, you are trying to solve some problem. Also, the input dataset of that problem is huge and you need to test the dataset over and over again. In that case you can use Python File Operation.
You can write the dataset in a text file and take input from that text file according to your need over and over again. Again, if you have to reuse the output of your program, you can save that in a file. Then, after finishing your program, you can analysis the output of that program using another program.
In these case you need Python File Operation. There may be some other cases where you may need Python File Operation.
According to the previous discussion, the first step we have to perform in Python File Operation is opening that file. You can open a file by using open function. This function take two arguments. The first one is file address and the other one is opening mode. There are some mode to open a file. Most common of them are listed below:. As Windows makes difference between binary file and text file.
Now we want to open that file.The python script we are going to create will delete a specific line starting with the word defined. You can change this value to any according to your requirement. Then open same file again and write data to it. Last step is to call this function deleteLine. It is very simple and I am not going to confuse you with any other steps.
First, open the file and get all your lines from the file. Then reopen the file in write mode and write your lines back, except for the line you want to delete:. The best and fastest option, rather than storing everything in a list and re-opening the file to write it, is in my opinion to re-write the file elsewhere.
This is a "fork" from Lother 's answer which I believe that should be considered the right answer. The issue with reading lines in first pass and making changes deleting specific lines in the second pass is that if you file sizes are huge, you will run out of RAM.
Instead, a better approach is to read lines, one by one, and write them into a separate file, eliminating the ones you don't need. Only CPU cycles show processing in progress. I liked the fileinput approach as explained in this answer: Deleting a line from a text file python. Say for example I have a file which has empty lines in it and I want to remove empty lines, here's how I solved it:. If you use Linux, you can try the following approach.
Suppose you have a text file named animal. I think if you read the file into a list, then do the you can iterate over the list to look for the nickname you want to get rid of. You can do it much efficiently without creating additional files, but you'll have to write the result back to the source file.
In general, you can't; you have to write the whole file again at least from the point of change to the end. Probably, you already got a correct answer, but here is mine. Instead of using a list to collect unfiltered data what readlines method doesI use two files. One is for hold a main data, and the second is for filtering the data when you delete a specific string. Here is a code:.That means files can be images, text documents, executables, and much more.
Most files are organized by keeping them in individual folders. In Python, a file is categorized as either text or binary, and the difference between the two file types is important. Text files are structured as a sequence of lines, where each line includes a sequence of characters.
How to extract specific portions of a text file using Python
This is what you know as code or syntax. It ends the current line and tells the interpreter a new one has begun. A backslash character can also be used, and it tells the interpreter that the next character — following the slash — should be treated as a new line.
A binary file is any type of file that is not a text file. In other words, they must be applications that can read and interpret binary. In order to open a file for writing or use in Python, you must rely on the built-in open function. As explained above, open will return a file object, so it is most commonly used with two arguments.
An argument is nothing more than a value that has been provided to a function, which is relayed when you call it. The second argument you see — mode — tells the interpreter and developer which way the file will be used. The current information stored within the file is also displayed — or printed — for us to view. Once this has been done, you can move on to call the objects functions. The two most common functions are read and write.
Naturally, if you open the text file — or look at it — using Python you will see only the text we told the interpreter to add. If you need to extract a string that contains all characters in the file, you can use the following method:. The output of that command will display all the text inside the file, the same text we told the interpreter to add earlier. For example, with the following code the interpreter will read the first five characters of stored data and return it as a string:.
If you want to read a file line by line — as opposed to pulling the content of the entire file at once — then you use the readline function. You would execute the readline function as many times as possible to get the data you were looking for. Each time you run the method, it will return a string of characters that contains a single line of information from the file.
If we wanted to return only the third line in the file, we would use this:. But what if we wanted to return every line in the file, properly separated? You would use the same function, only in a new form. This is called the file. Notice how each line is separated accordingly? Note that this is not the ideal way to show users the content in a file.
When you want to read — or return — all the lines from a file in a more memory efficient, and fast manner, you can use the loop over method. The advantage to using this method is that the related code is both simple and easy to read. This method is used to add information or content to an existing file.
To start a new line after you write data to the file, you can add an EOL character.
Obviously, this will amend our current file to include the two new lines of text. What this does is close the file completely, terminating resources in use, in turn freeing them up for the system to deploy elsewhere.In this tutorial we are going to learn about Python File Operations such as python read file, python write file, open file, delete file and copy file. Our previous tutorial was on Python Dictionary.
You can find that in this link. In the previous tutorial we used console to take input. Now, we will be taking input using file. That means, we will read from and write into files. To do so, we need to maintain some steps. Those are. Suppose, you are trying to solve some problem. Also, the input dataset of that problem is huge and you need to test the dataset over and over again.
In that case you can use Python File Operation. You can write the dataset in a text file and take input from that text file according to your need over and over again. Again, if you have to reuse the output of your program, you can save that in a file. Then, after finishing your program, you can analysis the output of that program using another program.
In these case you need Python File Operation. There may be some other cases where you may need Python File Operation. According to the previous discussion, the first step we have to perform in Python File Operation is opening that file. You can open a file by using open function. This function take two arguments. The first one is file address and the other one is opening mode. There are some mode to open a file. Most common of them are listed below:. As Windows makes difference between binary file and text file.
Now we want to open that file. However, the open filename, mode function returns a file object. With that file object you can proceed your further operation. There are some methods to read from and write to file.
I have an application that reads lines from a file and runs its magic on each line as it is read. Once the line is read and properly processed, I would like to delete the line from the file. A backup of the removed line is already being kept. I would like to do something like. This seems like a simple problem, but I would like to do it right rather than a whole lot of complicated seek and tell calls. After spending far to long on this problem I decided that everyone was probably right and this it just not a good way to do things.
It just seemed so elegant solution. You can't.Python: Sort File Line by Line
It is just not possible with actual text file implementations on current filesystems. Text files are sequential, because the lines in a text file can be of any length. Deleting a particular line would mean rewriting the entire file from that point on. To delete the second line you'd have to move the third and fourth lines' positions in the disk. The only way would be to store the third and fourth lines somewhere, truncate the file on the second line, and rewrite the missing lines.
If you know the size of every line in the text file, you can truncate the file in any position using. You're better off keeping a index into the file so that you can start where you stopped last, without destroying part of the file. Something like this would work :.
Truncating the file as you read it seems a bit extreme. What if your script has a bug that doesn't cause an error? In that case you'll want to restart at the beginning of your file. How about having your script print the line number it breaks on and having it take a line number as a parameter so you can tell it which line to start processing from? First of all, calling the operation truncate is probably not the best pick. If I understand the problem correctly, you want to delete everything up to the current position in file.
I would expect truncate to cut everything from the current position up to the end of the file. This is how the standard Python truncate method works, at least if I Googled correctly. Second, I am not sure it is wise to modify the file while iterating on in using the for loop. The file iterator supports in-place filteringwhich means it should be fairly simple to drop the processed lines afterwards.
A related post has what seems a good strategy to do that, see How can I run the first process from a list of processes stored in a file and immediately delete the first line as if the file was a queue and I called "pop"?Extracting text from a file is a common task in scripting and programmingand Python makes it easy.
In this guide, we'll discuss some simple ways to extract text from a file using the Python 3 programming language. In this guide, we'll be using Python version 3. Most systems come pre-installed with Python 2. While Python 2. Unless you have a specific reason to write or support legacy Python code, we recommend working in Python 3.
Reading and Writing Files in Python
For Microsoft Windows, Python 3 can be downloaded from the Python official website. When installing, make sure the "Install launcher for all users" and "Add Python to PATH" options are both checked, as shown in the image below. On Linux, you can install Python 3 with your package manager. For instance, on Debian or Ubuntuyou can install it with the following command:.
For macOSthe Python 3 installer can be downloaded from python. On Linux and macOS, the command to run the Python 3 interpreter is python3. On Windows, if you installed the launcher, the command is py. The commands on this page use python3 ; if you're on Windows, substitute py for python3 in all commands.
Running Python with no options starts the interactive interpreter. For more information about using the interpreter, see Python overview: using the Python interpreter. If you accidentally enter the interpreter, you can exit it using the command exit or quit.
First, let's read a text file. Let's say we're working with a file named lorem. In all the examples that follow, we work with text contained in this file. Feel free to copy and paste the latin text above into a text file, and save it as lorem. A Python program can read a text file using the built-in open function. For example, below is a Python 3 program that opens lorem. The " rt " parameter in the open function means "we're opening this file to r ead t ext data".
The hash mark " " means that everything on the rest of that line is a commentand it is ignored by the Python interpreter. If you save this program in a file called read. It's important to close your open files as soon as possible: open the file, perform your operation, and close it.
Don't leave it open for extended periods of time. When you're working with files, it's good practice to use the with open