How to Read a Line Character by Character in Assembly
How to extract specific portions of a text file using Python
Updated: 06/30/2020 by Calculator Hope
Extracting text from a file is a common task in scripting and programming, and Python makes it easy. In this guide, nosotros'll hash out some simple means to extract text from a file using the Python iii programming language.
Make certain you're using Python 3
In this guide, we'll be using Python version 3. Most systems come up pre-installed with Python 2.7. While Python ii.vii is used in legacy code, Python three is the present and future of the Python language. Unless you lot have a specific reason to write or support Python 2, we recommend working in Python 3.
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 together Python to PATH" options are both checked, equally shown in the image below.
On Linux, you can install Python three with your package director. For example, on Debian or Ubuntu, you can install information technology with the following command:
sudo apt-become update && sudo apt-get install python3
For macOS, the Python 3 installer can be downloaded from python.org, as linked above. If you are using the Homebrew package manager, it tin can too be installed by opening a terminal window (Applications → Utilities), and running this command:
brew install python3
Running Python
On Linux and macOS, the command to run the Python 3 interpreter is python3. On Windows, if you installed the launcher, the control is py. The commands on this page apply python3; if yous're on Windows, substitute py for python3 in all commands.
Running Python with no options starts the interactive interpreter. For more information well-nigh using the interpreter, see Python overview: using the Python interpreter. If y'all accidentally enter the interpreter, you can go out it using the command exit() or quit().
Running Python with a file name will interpret that python plan. For example:
python3 program.py
...runs the programme contained in the file program.py.
Okay, how can we use Python to extract text from a text file?
Reading data from a text file
First, let's read a text file. Let's say nosotros're working with a file named lorem.txt, which contains lines from the Lorem Ipsum example text.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit amet pretium tellus. Quisque at dignissim lacus.
Note
In all the examples that follow, we work with the four lines of text independent in this file. Copy and paste the latin text above into a text file, and salvage it as lorem.txt, so you can run the instance code using this file as input.
A Python plan can read a text file using the congenital-in open() function. For instance, the Python 3 program below opens lorem.txt for reading in text mode, reads the contents into a string variable named contents, closes the file, and prints the information.
myfile = open("lorem.txt", "rt") # open lorem.txt for reading text contents = myfile.read() # read the entire file to string myfile.close() # close the file print(contents) # impress cord contents Here, myfile is the proper noun we give to our file object.
The "rt" parameter in the open() function ways "we're opening this file to read text data"
The hash marker ("#") means that everything on that line is a annotate, and it's ignored past the Python interpreter.
If you lot save this program in a file called read.py, you can run it with the following control.
python3 read.py
The command higher up outputs the contents of lorem.txt:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit amet pretium tellus. Quisque at dignissim lacus.
Using "with open up"
It's of import to shut your open files equally before long as possible: open the file, perform your operation, and close it. Don't leave it open for extended periods of time.
When you lot're working with files, it's expert practice to use the with open...as chemical compound statement. It's the cleanest way to open a file, operate on it, and close the file, all in ane like shooting fish in a barrel-to-read block of code. The file is automatically closed when the code block completes.
Using with open up...equally, we can rewrite our programme to look like this:
with open ('lorem.txt', 'rt') equally myfile: # Open lorem.txt for reading text contents = myfile.read() # Read the entire file to a string print(contents) # Print the cord Annotation
Indentation is important in Python. Python programs employ white space at the beginning of a line to define scope, such equally a block of lawmaking. We recommend yous use four spaces per level of indentation, and that y'all utilize spaces rather than tabs. In the following examples, make certain your lawmaking is indented exactly as it'south presented hither.
Example
Relieve the programme as read.py and execute it:
python3 read.py
Output:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit amet pretium tellus. Quisque at dignissim lacus.
Reading text files line-by-line
In the examples so far, we've been reading in the whole file at one time. Reading a total file is no big bargain with small-scale files, but generally speaking, it'southward non a great idea. For one matter, if your file is bigger than the amount of available retention, y'all'll run across an error.
In nigh every case, it's a better idea to read a text file i line at a fourth dimension.
In Python, the file object is an iterator. An iterator is a type of Python object which behaves in certain ways when operated on repeatedly. For instance, yous tin can utilise a for loop to operate on a file object repeatedly, and each fourth dimension the aforementioned functioning is performed, you'll receive a different, or "next," effect.
Example
For text files, the file object iterates one line of text at a time. Information technology considers i line of text a "unit" of data, and so we tin use a for...in loop statement to iterate one line at a time:
with open ('lorem.txt', 'rt') as myfile: # Open lorem.txt for reading for myline in myfile: # For each line, read to a cord, print(myline) # and print the string. Output:
Lorem ipsum dolor sit down amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit amet pretium tellus. Quisque at dignissim lacus.
Find that we're getting an extra line break ("newline") later every line. That's because ii newlines are being printed. The showtime 1 is the newline at the stop of every line of our text file. The 2d newline happens because, past default, impress() adds a linebreak of its own at the end of any yous've asked it to print.
Let's store our lines of text in a variable — specifically, a listing variable — and then we can look at it more closely.
Storing text data in a variable
In Python, lists are like to, but non the same every bit, an assortment in C or Java. A Python listing contains indexed data, of varying lengths and types.
Example
mylines = [] # Declare an empty listing named mylines. with open ('lorem.txt', 'rt') equally myfile: # Open lorem.txt for reading text data. for myline in myfile: # For each line, stored as myline, mylines.append(myline) # add its contents to mylines. impress(mylines) # Print the list. The output of this program is a piffling different. Instead of printing the contents of the list, this program prints our list object, which looks similar this:
Output:
['Lorem ipsum dolor sit down amet, consectetur adipiscing elit.\n', 'Nunc fringilla arcu congue metus aliquam mollis.\n', 'Mauris nec maximus purus. Maecenas sit amet pretium tellus.\north', 'Quisque at dignissim lacus.\due north']
Hither, we see the raw contents of the listing. In its raw object form, a list is represented as a comma-delimited list. Here, each element is represented as a string, and each newline is represented every bit its escape grapheme sequence, \due north.
Much similar a C or Java assortment, the list elements are accessed past specifying an alphabetize number after the variable name, in brackets. Alphabetize numbers start at nothing — other words, the nth element of a list has the numeric alphabetize n-1.
Note
If you're wondering why the index numbers beginning at zero instead of one, you're non solitary. Calculator scientists have debated the usefulness of zero-based numbering systems in the past. In 1982, Edsger Dijkstra gave his opinion on the subject, explaining why cypher-based numbering is the all-time way to index data in computer science. You can read the memo yourself — he makes a compelling argument.
Example
Nosotros tin print the beginning element of lines by specifying alphabetize number 0, contained in brackets after the proper noun of the listing:
print(mylines[0])
Output:
Lorem ipsum dolor sit down amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis.
Example
Or the tertiary line, by specifying index number 2:
print(mylines[two])
Output:
Quisque at dignissim lacus.
Merely if we attempt to access an index for which in that location is no value, we go an error:
Example
print(mylines[3])
Output:
Traceback (about contempo call last): File <filename>, line <linenum>, in <module> print(mylines[3]) IndexError: list index out of range
Example
A list object is an iterator, and so to print every chemical element of the list, we tin can iterate over information technology with for...in:
mylines = [] # Declare an empty list with open ('lorem.txt', 'rt') as myfile: # Open lorem.txt for reading text. for line in myfile: # For each line of text, mylines.append(line) # add that line to the list. for chemical element in mylines: # For each element in the listing, print(element) # print it. Output:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit down amet pretium tellus. Quisque at dignissim lacus.
But we're still getting extra newlines. Each line of our text file ends in a newline graphic symbol ('\n'), which is being printed. Likewise, after printing each line, print() adds a newline of its own, unless you tell it to do otherwise.
We can change this default beliefs by specifying an end parameter in our print() phone call:
print(element, end='')
Past setting stop to an empty cord (2 unmarried quotes, with no space), we tell print() to impress null at the finish of a line, instead of a newline character.
Case
Our revised plan looks like this:
mylines = [] # Declare an empty list with open ('lorem.txt', 'rt') every bit myfile: # Open file lorem.txt for line in myfile: # For each line of text, mylines.suspend(line) # add together that line to the list. for chemical element in mylines: # For each element in the list, impress(element, end='') # print information technology without extra newlines. Output:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis. Mauris nec maximus purus. Maecenas sit amet pretium tellus. Quisque at dignissim lacus.
The newlines you encounter here are actually in the file; they're a special character ('\n') at the end of each line. We want to become rid of these, and then we don't have to worry nearly them while we process the file.
How to strip newlines
To remove the newlines completely, we can strip them. To strip a cord is to remove one or more characters, normally whitespace, from either the beginning or end of the cord.
Tip
This procedure is sometimes also called "trimming."
Python 3 string objects have a method called rstrip(), which strips characters from the correct side of a string. The English language reads left-to-right, so stripping from the right side removes characters from the stop.
If the variable is named mystring, we can strip its right side with mystring.rstrip(chars), where chars is a string of characters to strip. For case, "123abc".rstrip("bc") returns 123a.
Tip
When y'all represent a string in your program with its literal contents, it'southward called a cord literal. In Python (as in most programming languages), string literals are always quoted — enclosed on either side by unmarried (') or double (") quotes. In Python, single and double quotes are equivalent; you lot can use one or the other, as long as they lucifer on both ends of the string. It's traditional to represent a homo-readable string (such as Hello) in double-quotes ("Hello"). If you lot're representing a single grapheme (such as b), or a unmarried special graphic symbol such as the newline character (\north), it's traditional to use unmarried quotes ('b', '\north'). For more data about how to use strings in Python, yous can read the documentation of strings in Python.
The argument cord.rstrip('\northward') will strip a newline grapheme from the correct side of cord. The following version of our program strips the newlines when each line is read from the text file:
mylines = [] # Declare an empty list. with open ('lorem.txt', 'rt') every bit myfile: # Open lorem.txt for reading text. for myline in myfile: # For each line in the file, mylines.append(myline.rstrip('\n')) # strip newline and add to list. for element in mylines: # For each element in the list, impress(element) # print it. The text is at present stored in a list variable, so individual lines tin can be accessed past index number. Newlines were stripped, so nosotros don't have to worry about them. We can always put them back later if nosotros reconstruct the file and write information technology to deejay.
Now, permit'due south search the lines in the listing for a specific substring.
Searching text for a substring
Let's say nosotros desire to locate every occurrence of a sure phrase, or even a single letter. For instance, maybe we need to know where every "due east" is. We can accomplish this using the string's find() method.
The list stores each line of our text as a cord object. All string objects take a method, observe(), which locates the first occurrence of a substrings in the cord.
Let'south utilise the find() method to search for the letter "e" in the offset line of our text file, which is stored in the listing mylines. The first element of mylines is a string object containing the commencement line of the text file. This string object has a find() method.
In the parentheses of discover(), we specify parameters. The first and but required parameter is the string to search for, "e". The statement mylines[0].find("eastward") tells the interpreter to search forward, starting at the kickoff of the string, one grapheme at a time, until it finds the letter "east." When information technology finds one, it stops searching, and returns the alphabetize number where that "e" is located. If information technology reaches the stop of the cord, information technology returns -1 to signal nothing was institute.
Example
impress(mylines[0].observe("east")) Output:
3
The return value "3" tells united states that the letter "e" is the quaternary character, the "due east" in "Lorem". (Remember, the index is naught-based: alphabetize 0 is the offset character, one is the second, etc.)
The find() method takes two optional, additional parameters: a start index and a stop alphabetize, indicating where in the string the search should brainstorm and end. For instance, cord.detect("abc", ten, 20) searches for the substring "abc", simply only from the 11th to the 21st character. If cease is not specified, find() starts at index start, and stops at the end of the cord.
Example
For instance, the post-obit statement searchs for "e" in mylines[0], beginning at the fifth grapheme.
print(mylines[0].find("east", four)) Output:
24
In other words, starting at the fifth character in line[0], the first "e" is located at index 24 (the "due east" in "nec").
Instance
To start searching at index ten, and stop at alphabetize xxx:
print(mylines[i].discover("east", 10, xxx)) Output:
28
(The first "e" in "Maecenas").
If detect() doesn't locate the substring in the search range, it returns the number -1, indicating failure:
print(mylines[0].find("e", 25, 30)) Output:
-1
There were no "due east" occurrences betwixt indices 25 and xxx.
Finding all occurrences of a substring
But what if we want to locate every occurrence of a substring, not merely the first one we encounter? We tin iterate over the string, starting from the alphabetize of the previous lucifer.
In this instance, nosotros'll use a while loop to repeatedly find the letter "e". When an occurrence is found, we call find again, starting from a new location in the cord. Specifically, the location of the final occurrence, plus the length of the cord (so nosotros can motility forward past the last one). When find returns -1, or the start index exceeds the length of the string, we stop.
# Build assortment of lines from file, strip newlines mylines = [] # Declare an empty list. with open ('lorem.txt', 'rt') as myfile: # Open up lorem.txt for reading text. for myline in myfile: # For each line in the file, mylines.append(myline.rstrip('\north')) # strip newline and add together to list. # Locate and print all occurences of letter "due east" substr = "east" # substring to search for. for line in mylines: # cord to be searched index = 0 # current index: grapheme existence compared prev = 0 # previous index: final character compared while index < len(line): # While index has not exceeded cord length, alphabetize = line.discover(substr, index) # ready alphabetize to first occurrence of "eastward" if index == -ane: # If nothing was found, break # exit the while loop. print(" " * (index - prev) + "due east", end='') # impress spaces from previous # match, so the substring. prev = alphabetize + len(substr) # remember this position for next loop. index += len(substr) # increment the alphabetize by the length of substr. # (Repeat until index > line length) print('\n' + line); # Print the original string under the e's Output:
e e east e east Lorem ipsum dolor sit amet, consectetur adipiscing elit. due east due east Nunc fringilla arcu congue metus aliquam mollis. east e e e e e Mauris nec maximus purus. Maecenas sit amet pretium tellus. e Quisque at dignissim lacus.
Incorporating regular expressions
For circuitous searches, use regular expressions.
The Python regular expressions module is chosen re. To utilize it in your plan, import the module earlier you employ information technology:
import re
The re module implements regular expressions by compiling a search pattern into a pattern object. Methods of this object tin then be used to perform lucifer operations.
For case, let's say you want to search for any word in your document which starts with the letter of the alphabet d and ends in the letter r. We tin can attain this using the regular expression "\bd\west*r\b". What does this hateful?
| character sequence | pregnant |
|---|---|
| \b | A give-and-take boundary matches an empty string (anything, including nothing at all), but just if it appears earlier or subsequently a non-give-and-take grapheme. "Word characters" are the digits 0 through ix, the lowercase and capital letter letters, or an underscore ("_"). |
| d | Lowercase letter of the alphabet d. |
| \westward* | \westward represents whatsoever give-and-take character, and * is a quantifier meaning "aught or more than of the previous graphic symbol." So \w* will match nada or more discussion characters. |
| r | Lowercase alphabetic character r. |
| \b | Word boundary. |
And then this regular expression volition lucifer whatever cord that can be described as "a word boundary, then a lowercase 'd', then zero or more than word characters, then a lowercase 'r', then a discussion boundary." Strings described this way include the words destroyer, dour, and doctor, and the abbreviation dr.
To apply this regular expression in Python search operations, we first compile it into a pattern object. For case, the following Python statement creates a pattern object named design which nosotros tin can use to perform searches using that regular expression.
pattern = re.compile(r"\bd\west*r\b")
Note
The letter r before our string in the statement above is of import. Information technology tells Python to interpret our string as a raw string, exactly every bit we've typed information technology. If nosotros didn't prefix the string with an r, Python would translate the escape sequences such as \b in other ways. Whenever yous need Python to translate your strings literally, specify it as a raw string by prefixing it with r.
Now we tin can use the blueprint object's methods, such as search(), to search a string for the compiled regular expression, looking for a match. If it finds 1, it returns a special upshot called a friction match object. Otherwise, it returns None, a built-in Python abiding that is used like the boolean value "false".
import re str = "Good morning, doctor." pat = re.compile(r"\bd\west*r\b") # compile regex "\bd\westward*r\b" to a pattern object if pat.search(str) != None: # Search for the pattern. If institute, impress("Plant it.") Output:
Plant information technology.
To perform a case-insensitive search, yous tin specify the special constant re.IGNORECASE in the compile footstep:
import re str = "Hello, Physician." pat = re.compile(r"\bd\w*r\b", re.IGNORECASE) # upper and lowercase will match if pat.search(str) != None: print("Found it.") Output:
Found it.
Putting it all together
So now nosotros know how to open a file, read the lines into a list, and locate a substring in any given list element. Let's utilize this knowledge to build some example programs.
Print all lines containing substring
The plan below reads a log file line by line. If the line contains the give-and-take "error," it is added to a listing called errors. If not, it is ignored. The lower() string method converts all strings to lowercase for comparison purposes, making the search case-insensitive without altering the original strings.
Note that the discover() method is called directly on the result of the lower() method; this is called method chaining. Too, annotation that in the print() statement, we construct an output cord by joining several strings with the + operator.
errors = [] # The list where nosotros will store results. linenum = 0 substr = "error".lower() # Substring to search for. with open ('logfile.txt', 'rt') as myfile: for line in myfile: linenum += i if line.lower().find(substr) != -i: # if case-insensitive lucifer, errors.append("Line " + str(linenum) + ": " + line.rstrip('\n')) for err in errors: print(err) Input (stored in logfile.txt):
This is line i This is line 2 Line iii has an error! This is line 4 Line v besides has an error!
Output:
Line 3: Line iii has an error! Line 5: Line 5 also has an fault!
Extract all lines containing substring, using regex
The programme beneath is similar to the in a higher place program, merely using the re regular expressions module. The errors and line numbers are stored equally tuples, e.thousand., (linenum, line). The tuple is created by the additional enclosing parentheses in the errors.append() argument. The elements of the tuple are referenced similar to a list, with a zilch-based index in brackets. As synthetic here, err[0] is a linenum and err[i] is the associated line containing an fault.
import re errors = [] linenum = 0 pattern = re.compile("error", re.IGNORECASE) # Compile a case-insensitive regex with open ('logfile.txt', 'rt') every bit myfile: for line in myfile: linenum += one if blueprint.search(line) != None: # If a lucifer is found errors.append((linenum, line.rstrip('\northward'))) for err in errors: # Iterate over the list of tuples print("Line " + str(err[0]) + ": " + err[1]) Output:
Line half-dozen: Mar 28 09:10:37 Error: cannot contact server. Connection refused. Line 10: Mar 28 10:28:fifteen Kernel error: The specified location is not mounted. Line 14: Mar 28 11:06:thirty Fault: usb 1-1: can't gear up config, exiting.
Extract all lines containing a telephone number
The programme below prints whatever line of a text file, info.txt, which contains a US or international phone number. It accomplishes this with the regular expression "(\+\d{1,ii})?[\s.-]?\d{3}[\southward.-]?\d{four}". This regex matches the following phone number notations:
- 123-456-7890
- (123) 456-7890
- 123 456 7890
- 123.456.7890
- +91 (123) 456-7890
import re errors = [] linenum = 0 pattern = re.compile(r"(\+\d{i,two})?[\south.-]?\d{3}[\s.-]?\d{iv}") with open ('info.txt', 'rt') as myfile: for line in myfile: linenum += 1 if design.search(line) != None: # If pattern search finds a match, errors.suspend((linenum, line.rstrip('\n'))) for err in errors: print("Line ", str(err[0]), ": " + err[1]) Output:
Line 3 : My phone number is 731.215.8881. Line 7 : Y'all can attain Mr. Walters at (212) 558-3131. Line 12 : His agent, Mrs. Kennedy, tin exist reached at +12 (123) 456-7890 Line fourteen : She tin can also be contacted at (888) 312.8403, extension 12.
Search a dictionary for words
The plan below searches the dictionary for any words that start with h and end in pe. For input, it uses a dictionary file included on many Unix systems, /usr/share/dict/words.
import re filename = "/usr/share/dict/words" design = re.compile(r"\bh\w*pe$", re.IGNORECASE) with open(filename, "rt") equally myfile: for line in myfile: if blueprint.search(line) != None: print(line, end='')
Output:
Hope heliotrope promise hornpipe horoscope hype
Source: https://www.computerhope.com/issues/ch001721.htm
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