Python 3 replace line in file

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Python 3 replace line in file

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Sometimes the new hex address is 5 characters long. It seems the new hex address is written correctly, but the first character of the next line erased.

python 3 replace line in file

Any ideas why? There are many ways to solve this problem. Many times just writing a simpler algorithm can solve your problem as there are less pitfalls for error. In my example we simply iterate through each line of the input file, if the line matches the criteria to replace, then we replace, and write it in our new file.

Otherwise we leave the line the same and write it to the new file. Nice and simple. Learn more. Replace line in.

Asked yesterday. Active yesterday. Viewed 41 times. Im new to python and got stuck while processing a. Sometimes the new hex address is 5 characters long 4 Below is the python code I wrote to do this: write LVMain start address in. Alex Alex 1. New contributor. This way of iterating over the lines is way too complicated and easily leads to mistakes such as what you have encountered here. You should just use for line in file and output either the original or a modified line to a new file. I know nothing about python, but it seems you don't work with lines, but with byte positions.

So when you write a line that is longer than the original, it will overwrite the start of the next line.

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Does this answer your question? Active Oldest Votes. Meathusk Meathusk 21 3 3 bronze badges. While this code may solve the question, including an explanation of how and why this solves the problem would really help to improve the quality of your post, and probably result in more up-votes.Opening a file and reading the content of a file is one of the common things you would do while doing data analysis. We first open the file using open function in read only mode.

And use the file handler from opening the file to read all lines using readlines as follows. We can also read all the lines of a file at once in another way. Basically, we would use the file handler object after opening the file as argument to list function to get all the lines as a list.

However, it is not memory efficient to use if your text files are really big. A better way to read a text file that is memory-friendly is to read the file line by line, that is one line at a time.

Since we read one line at a time with readlinewe can easily handle big files without worrying about memory problems. Another variation of reading a file with while statement and readline statement is as follows. Here the while tests for boolean and read line by line until we reach the end of file and line will be empty.

One can check if a file handler is closed with. The with operator creates a context manager and it will automatically close the file for you when you are done with it. Do you have data in a csv or tab limited text file and want to read it in Python?

Here is how to load data files in python with Pandas. Email Address. Share this: Twitter Facebook. Related posts: How to read entire text file in Python?

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Python Cookbook by Alex Martelli, David Ascher

Manage your Cookies Settings. Join Now login. Ask Question. Hii am in the middle of creating the script to match the string and replace it in mount. Jun 4 ' Post Reply. Share this Question.

Expert Mod 2. Hello DannyMc, string. You must make an assignment. Unless your file is very large, I would suggest updating the text in the file by reading the file, modifying the text, and writing the modified text back to the file. You can use string method replace instead of importing the string module. Expand Select Wrap Line Numbers. Jun 5 ' Jun 11 ' Jun 25 ' I am a bit unclear about what mount. You do not need re. You can test for membership with the in operator.

Works nice, thanks! Aug 9 ' My file extensions are. Nov 4 ' Similar topics search text file Pop up calendar error. Search Text File Fastest way to search text file for string.

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Question stats viewed: replies: 7 date asked: Jun 4 '09 Follow this discussion. Follow us to get the Latest Bytes Updates.

Python to search text file string and replace it P: DannyMc Hii am in the middle of creating the script to match the string and replace it in mount. Thank you! I am using CentOS 4. P: DannyMc thanks for the solution bvdet!! Jun 5 '09 reply. Is there anyway to simplify it???

Jun 25 '09 reply. Aug 9 '17 reply.Get the latest tutorials on SysAdmin and open source topics. Write for DigitalOcean You get paid, we donate to tech non-profits. DigitalOcean Meetups Find and meet other developers in your city. Become an author. Python is a great tool for processing data. It is likely that any program you write will involve reading, writing, or manipulating data. For example, consider a Python program that checks a list of users for access control.

Your list of users will likely be stored and saved in a text file. Perhaps you are not working with text, but instead have a program that does financial analysis. In order to do some number crunching, you will likely have to input those numbers from a saved spreadsheet.

Python File Handling: Create, Open, Append, Read, Write

Regardless of your application, it is almost guaranteed that inputting or outputting data will be involved. This tutorial will briefly describe some of the format types Python is able to handle. For this tutorial, you should have Python 3 installed as well as a local programming environment set up on your computer. If this is not the case, you can get set up by following the appropriate installation and set up guide for your operating system:.

Python is super accommodating and can, with relative ease, handle a number of different file formats, including but not limited to the following:. Before we can begin working in Python, we need to make sure we have a file to work with. In the new file, enter a few lines of text.

Next, save your file and make sure you know where you put it. This will be very important in later steps, where we open the file in Python. Before we can write our program, we have to create a Python programming file, so create the file files. To make things easy, save it in the same directory as our days.

To open a file in Python, we first need some way to associate the file on disk with a variable in Python. This process is called opening a file. We begin by telling Python where the file is. The location of your file is often referred to as the file path. In order for Python to open your file, it requires the path.

The path to our days.

How to delete a line in text file using Python

In Python, we will create a string variable to store this information. In our files. The open function requires as its first argument the file path. The function also allows for many other parameters. However, most important is the optional mode parameter. Mode is an optional string that specifies the mode in which the file is opened.

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The mode you choose will depend on what you wish to do with the file. Here are some of our mode options:. In this example, we only want to read from the file, so we will use the 'r' mode.I want to loop over the contents of a text file and do a search and replace on some lines and write the result back to the file.

I could first load the whole file in memory and then write it back, but that probably is not the best way to do it.

I guess something like this should do it. It basically writes the content to a new file and replaces the old file with the new file:. The shortest way would probably be to use the fileinput module. For example, the following adds line numbers to a file, in-place:.

python 3 replace line in file

For example, it can be used to automatically operate on all files in sys. Starting with Python 3. While fileinput is great for throwaway scripts, I would be wary of using it in real code because admittedly it's not very readable or familiar. In real production code it's worthwhile to spend just a few more lines of code to make the process explicit and thus make the code readable.

Based on the answer by Thomas Watnedal. However, this does not answer the line-to-line part of the original question exactly. The function can still replace on a line-to-line basis.

This implementation replaces the file contents without using temporary files, as a consequence file permissions remain unchanged. Reading the file as a single string instead of line by line allows for multiline match and replacement. If you're wanting a generic function that replaces any text with some other text, this is likely the best way to go, particularly if you're a fan of regex's:.

Create a new file, copy lines from the old to the new, and do the replacing before you write the lines to the new file. Expanding on Kiran's answer, which I agree is more succinct and Pythonic, this adds codecs to support the reading and writing of UTF See below for example.

Using hamishmcn's answer as a template I was able to search for a line in a file that match my regex and replacing it with empty string. Why am I getting the errorwhen the fetch url is ok? Dot notation in React imports. How to fix after attempt to override existing POST? Dynamic Object Reference in Object. Altering element's content in javascript.

I have plotted a horizontal bar graph with a large number of horizontal barsIs there any way I could prevent the overlapping of the y axis titles and the bars in the bar graphs. I have a service which runs in terminal like a messenger and it has its own printing console like so:. What is the best way to do this, within the following code? Home Python Search and replace a line in a file in Python. How to use orthogonal data to sort in DataTables? User meta validation and retrival.By using this site, you agree to our updated Privacy Policy and our Terms of Use.

Manage your Cookies Settings. Join Now login. Ask Question. How do I change a line in a file?? For example I have the follwing text in my file: line1 line2 line3 line4 How do I replace 'line2' with 'newline'. Since the write operation in python will overwrite everything.

Regards, Kah. Jul 19 ' Post Reply. Share this Question. Michael Hoffman. Hi, the example provided by Vishnu is quite close to what I want but it still required me to write all the data back to the file. Is there any way where I can just write a particular line? Steve Holden. If you are asking whether you can update a file in place, the answer is "yes" - look up "lseek" in the Python documentation module os, under "File Descriptor Operations".

However, you asked about replacing one line with another of a different length: since this will mean changing the offsets of all subsequent bytes you have no way to do this other than writing out the whole content of the file following the modification. You would also have to ensure that you truncated the file to the correct length.

Find and Replace Method : Python tutorial 34

In general, although they don't make it obvious that they are doing so most programs that "change" files text editors and the like are really writing new copies. Miki Tebeka. Hello kah, How do I change a line in a file?? Larry Bates. You might be able to use "edge" case to make this simple.

python 3 replace line in file

Larry Bates kah wrote: How do I change a line in a file?? Kirk Job Sluder. In addition, I would argue that editing a file in place using a non-interactive program is dangerous and bad practice in general. By the time you find a bug in your edit script, the original is lost. This is something I learned from bitter experience when I tried to be smart and make script-based edits over entire directories of html files.

In unix shell scripting idiom, I would do something like: mv file file.After this, you can adopt one of these methods in your projects that fits the best as per conditions. It would depend on many parameters, such as the frequency of such an operation, the size of the file, etc. We recommend this solution for files with a smaller size. If the file size is large, then it becomes inefficient as it loads the entire file in memory. The readlines returns a sequence of all lines from the file each containing newline char except the last one.

Here, you can see that we are also using the Python while loop to traverse the lines. On the other hand, the above solution will cause high memory usage for large files.

So, you should choose a different approach. When the file size reaches to MBs or in GB, then the right idea is to get one line at a time. Python readline method does this job efficiently. It does not load all data in one go. The readline reads the text until the newline character and returns the line.

It handles the EOF end of file by returning a blank string. Python provides a concept of context managers. It keeps track of the open file and closes it automatically after the file operation ends. Hence, we can confirm that you never miss closing a filehandle using the context manager. It performs cleanup tasks like closing a file accordingly. In the below example, you can see that we are using the context manager with along with the for loop to first write and then reading lines.

Contents 1 Readlines to read all lines together 1. Next Post Python f-string for String Formatting. September 20,


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