Reading one line in python

Как прочитать файл построчно в Python

Существует много способов чтение из файла построчно в Python. Вы можете считать строки в список или обращаться к каждой из строк в цикле при помощи итератора или вызова функции объекта file.

В этом руководстве мы научимся считывать файл построчно, используя функции readline() , readlines() и объект файла на примерах различных программ.

Пример 1: Чтение файла построчно функцией readline()

В этом примере мы будем использовать функцию readline() для файлового объекта, получая каждую строку в цикле.

Как использовать функцию file.readline()

Следуйте пунктам приведенным ниже для того, чтобы считать файл построчно, используя функцию readline() .

1. Открываем файл в режиме чтения. При этом возвращается дескриптор файла.
2. Создаём бесконечный цикл while.
   1. В каждой итерации считываем строку файла при помощи readline() .
   2. Если строка не пустая, то выводим её и переходим к следующей. Вы можете проверить это, используя конструкцию if not . В противном случае файл больше не имеет строк и мы останавливаем цикл с помощью break .

    
    
   # получим объект файла file1 = open("sample.txt", "r") while True: # считываем строку line = file1.readline() # прерываем цикл, если строка пустая if not line: break # выводим строку print(line.strip()) # закрываем файл file1.close
    
   Привет! Добро пожаловать на PythonRu. Удачи в обучении!
    
   Пример 2: Чтение строк как список функцией readlines()
    
   Функция readlines() возвращает все строки файла в виде списка. Мы можем пройтись по списку и получить доступ к каждой строке.
    
   В следующей программе мы должны открыть текстовый файл и получить список всех его строк, используя функцию readlines() . После этого мы используем цикл for, чтобы обойти данный список.
    
    
   # получим объект файла file1 = open("sample.txt", "r") # считываем все строки lines = file1.readlines() # итерация по строкам for line in lines: print(line.strip()) # закрываем файл file1.close
    
   Привет! Добро пожаловать на PythonRu. Удачи в обучении!
    
   Пример 3: Считываем файл построчно из объекта File
    
   В нашем первом примере, мы считываем каждую строку файла при помощи бесконечного цикла while и функции readline() . Но Вы можете использовать цикл for для файлового объекта, чтобы в каждой итерации цикла получать строку, пока не будет достигнут конец файла.
    
   Ниже приводится программа, демонстрирующая применение оператора for-in, для того, чтобы перебрать строки файла.
    
   Для демонстрации откроем файл с помощью with open. Это применимо и к предыдущим двум примерам.
    
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   3 Ways to Read A Text File Line by Line in Python
    
    
   Opening a file and reading the content of a file is one of the common things you would do while doing data analysis.
    
   In this tutorial, we will see 3 examples of reading a text file in Python 3.
    
   One easy way to read a text file and parse each line is to use the python statement “readlines” on a file object.
    
   How To Read all lines in a file at once? Use readlines()
    
   If you want to read all lines of a file at the same time, Python’s readlines() function is for you. Python’s readlines function reads everything in the text file and has them in a list of lines. Here is an example of how to use Python’s readlines.
    
   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.
    
   # Open the file with read only permit f = open('my_text_file.txt', "r") # use readlines to read all lines in the file # The variable "lines" is a list containing all lines in the file lines = f.readlines() # close the file after reading the lines. f.close()
    
   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.
    
   Another way to read lines at once is to simply use
    
   # read all lines at once lines = list(f)
    
   Note that the last character of each line is newline character.
    
   Then you can go over the list of “lines” to parse each line. As you can immediately notice, “readlines” or “list(f) works great for a small text file. 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.
    
   Core Python has (at least) two ways to read a text file line by line easily.
    
   How To Read a Text File Line by Line Using While Statement in Python?
    
   Here is the way to read text file one line at a time using “While” statement and python’s readline function. Since we read one line at a time with readline, we can easily handle big files without worrying about memory problems.
    
   # Open the file with read only permit f = open('my_text_file.txt') # use readline() to read the first line line = f.readline() # use the read line to read further. # If the file is not empty keep reading one line # at a time, till the file is empty while line: # in python 2+ # print line # in python 3 print is a builtin function, so print(line) # use realine() to read next line line = f.readline() f.close()
    
   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.
    
   # file handle fh fh = open('my_text_file.txt') while True: # read line line = fh.readline() # in python 2, print line # in python 3 print(line) # check if line is not empty if not line: break fh.close()
    
   How To Read a Text File Line by Line Using an Iterator in Python?
    
   One can also use an iterator to read a text file one line at time. Here is how to do it.
    
   fh = open('my_text_file.txt') for line in fh: # in python 2 # print line # in python 3 print(line) fh.close()
    
   Remembering to close the file handler (“fh”) with the statement “fh.close()” can be difficult initially. One can check if a file handler is closed with
    
   # check if the file file handler is closed or not >fh.closed # true if the file handler is closed True
    
   One can open files in a much simpler way using “with” statement in Python, without having to close the file handler. The with operator creates a context manager and it will automatically close the file for you when you are done with it.
    
   Check here to see how to use “with” statement to open file.
    
   Do you want to read a text file line by line and skip comment lines?, check this post
    
   Do you want to read/load text file numerical data, check this post
    
   Do you have data in a csv or tab limited text file and want to read it in Python? The best option is to use Python’s pandas package. Here is how to load data files in python with Pandas,
    
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   Python Open File – How to Read a Text File Line by Line
    
    
   In Python, there are a few ways you can read a text file.
    
   In this article, I will go over the open() function, the read() , readline() , readlines() , close() methods, and the with keyword.
    
   What is the open() function in Python?
    
   If you want to read a text file in Python, you first have to open it.
    
   This is the basic syntax for Python's open() function:
    
   open("name of file you want opened", "optional mode")
    
   File names and correct paths
    
   If the text file and your current file are in the same directory ("folder"), then you can just reference the file name in the open() function.
    
   Here is an example of both files being in the same directory:
    
    
   If your text file is in a different directory, then you will need to reference the correct path name for the text file.
    
   In this example, the random-text file is inside a different folder then main.py :
    
    
   In order to access that file in the main.py , you have to include the folder name with the name of the file.
    
   open("text-files/random-text.txt")
    
   If you don't have the correct path for the file, then you will get an error message like this:
    
    
   It is really important to keep track of which directory you are in so you can reference the correct path name.
    
   Optional Mode parameter in open()
    
   There are different modes when you are working with files. The default mode is the read mode.
    
   The letter r stands for read mode.
    
   You can also omit mode= and just write "r" .
    
   There are other types of modes such as "w" for writing or "a" for appending. I am not going to go into detail for the other modes because we are just going to focus on reading files.
    
   For a complete list of the other modes, please read through the documentation.
    
   Additional parameters for the open() function in Python
    
   The open() function can take in these optional parameters.
    
   To learn more about these optional parameters, please read through the documentation.
    
   What is the readable() method in Python?
    
   If you want to check if a file can be read, then you can use the readable() method. This will return a True or False .
    
   This example would return True because we are in the read mode:
    
   file = open("demo.txt") print(file.readable())
    
    
   If I changed this example, to "w" (write) mode, then the readable() method would return False :
    
   file = open("demo.txt", "w") print(file.readable())
    
    
   What is the read() method in Python?
    
   The read() method is going to read all of the content of the file as one string. This is a good method to use if you don't have a lot of content in the text file.
    
   In this example, I am using the read() method to print out a list of names from the demo.txt file:
    
   file = open("demo.txt") print(file.read())
    
    
   This method can take in an optional parameter called size. Instead of reading the whole file, only a portion of it will be read.
    
   If we modify the earlier example, we can print out only the first word by adding the number 4 as an argument for read() .
    
   file = open("demo.txt") print(file.read(4))
    
    
   If the size argument is omitted, or if the number is negative, then the whole file will be read.
    
   What is the close() method in Python?
    
   Once you are done reading a file, it is important that you close it. If you forget to close your file, then that can cause issues.
    
   This is an example of how to close the demo.txt file:
    
   file = open("demo.txt") print(file.read()) file.close()
    
   How to use the with keyword to close files in Python
    
   One way to ensure that your file is closed is to use the with keyword. This is considered good practice, because the file will close automatically instead of you having to manually close it.
    
   Here is how to rewrite our example using the with keyword:
    
   with open("demo.txt") as file: print(file.read())
    
   What is the readline() method in Python?
    
   This method is going to read one line from the file and return that.
    
   In this example, we have a text file with these two sentences:
    
   This is the first line This is the second line
    
   If we use the readline() method, it will only print the first sentence of the file.
    
   with open("demo.txt") as file: print(file.readline())
    
    
   This method also takes in the optional size parameter. We can modify the example to add the number 7 to only read and print out This is :
    
   with open("demo.txt") as file: print(file.readline(7))
    
    
   What is the readlines() method in Python?
    
   This method will read and return a list of all of the lines in the file.
    
   In this example, we are going to print out our grocery items as a list using the readlines() method.
    
   with open("demo.txt") as file: print(file.readlines())
    
    
   How to use a for loop to read lines from a file in Python
    
   An alternative to these different read methods would be to use a for loop .
    
   In this example, we can print out all of the items in the demo.txt file by looping over the object.
    
   with open("demo.txt") as file: for item in file: print(item)
    
    
   Conclusion
    
   If you want to read a text file in Python, you first have to open it.
    
   open("name of file you want opened", "optional mode") 
    
   If the text file and your current file are in the same directory ("folder"), then you can just reference the file name in the open() function.
    
   If your text file is in a different directory, then you will need to reference the correct path name for the text file.
    
   The open() function takes in the optional mode parameter. The default mode is the read mode.
    
   If you want to check if a file can be read, then you can use the readable() method. This will return a True or False .
    
   The read() method is going to read all of the content of the file as one string.
    
   Once you are done reading a file, it is important that you close it. If you forget to close your file, then that can cause issues.
    
   One way to ensure that your file is closed is to use the with keyword.
    
   with open("demo.txt") as file: print(file.read())
    
   The readline() method is going to read one line from the file and return that.
    
   The readlines() method will read and return a list of all of the lines in the file.
    
   An alternative to these different read methods would be to use a for loop .
    
   with open("demo.txt") as file: for item in file: print(item)
    
   I hope you enjoyed this article and best of luck on your Python journey. 
    
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