Python Read the First Half of File

Reading files line-by-line

Topic Series: Files

Watch as video

01:52

Allow'due south talk about reading files line-by-line in Python.

Looping over file objects to read line-by-line

Here we're calling the read method on a file object (for a file called diary980.md):

                                      >>>                            filename              =              "diary980.md"              >>>                            with              open              (              filename              )              as              diary_file              :              ...                            contents              =              diary_file              .              read              ()              ...              >>>                            contents              'Python Log -- Day 980\n\nToday I learned about metaclasses.\nMetaclasses are a course\'south class.\nMeaning every class is an instance of a metaclass.\nThe default metaclass is "blazon".\north\nClasses command features (like string representations) of all their instances.\nMetaclasses can control similar features for their classes.\n\nI doubt I\'ll ever need to make a metaclass, at least not for production code.\north'                      

When y'all call the read method on a file object, Python will read the entire file into memory all at once. But that could be a bad idea if yous're working with a really big file.

In that location's another common fashion to process files in Python: y'all can loop over a file object to read it line-by-line:

                                      >>>                            filename              =              "diary980.md"              >>>                            with              open              (              filename              )              as              diary_file              :              ...                            n              =              1              ...                            for              line              in              diary_file              :              ...                            print              (              n              ,              line              )              ...                            northward              +=              1              ...                      

Here, we're printing out a number (counting upwards) in each line in our file:

                                      one Python Log -- Day 980              two              three Today I learned about metaclasses.              four Metaclasses are a class's class.              v Meaning every class is an example of a metaclass.              six The default metaclass is "blazon".              7              viii Classes command features (like string representations) of all their instances.              9 Metaclasses tin control similar features for their classes.              10              xi I dubiousness I'll ever need to make a metaclass, at least not for product code.                      

Notice that every bit we print, Python isn't just press out the line, but an actress blank line in between each line in our file. By default, Python'due south print function prints a newline character (\north) after whatever else that it prints (see the print function's end argument). Just each of our lines besides stop in a newline character, because newline characters are what separate lines in a file:

                                      >>>                            line              "I dubiousness I'll ever demand to brand a metaclass, at least not for production lawmaking.\n"                      

Getting rid of the newline graphic symbol when reading line-by-line

So nosotros either need to suppress the newline character that the print role prints out or we need to remove the newline characters from each line in our file as we print them out:

                                      >>>                            filename              =              "diary980.md"              >>>                            with              open              (              filename              )              as              diary_file              :              ...                            n              =              1              ...                            for              line              in              diary_file              :              ...                            impress              (              n              ,              line              .              rstrip              (              "              \n              "              ))              ...                            n              +=              1              ...              1 Python Log -- Day 980              two              iii Today I learned most metaclasses.              4 Metaclasses are a class's grade.              5 Significant every form is an instance of a metaclass.              6 The default metaclass is "type".              vii              8 Classes command features (like string representations) of all their instances.              9 Metaclasses can control similar features for their classes.              10              11 I doubt I'll ever need to make a metaclass, at least non for production code.                      

We're using the string lstrip method here to "strip" newline characters from the left-hand side (the beginning) of each of our line strings just earlier impress each line.

File objects are lazy iterables

File objects in Python are lazy iterables, which means we can treat them pretty much the same way equally any other iterable.

So instead of manually counting upwards, we could pass our file object to the congenital-in enumerate part. The enumerate function could so do the counting for us every bit nosotros loop:

                                      >>>                            filename              =              "diary980.md"              >>>                            with              open              (              filename              )              as              diary_file              :              ...                            for              due north              ,              line              in              enumerate              (              diary_file              ,              get-go              =              1              ):              ...                            print              (              n              ,              line              .              rstrip              (              '              \n              '              ))                      

Nosotros've remove two lines of code but we get the same output as before:

                        one Python Log -- Day 980 two 3 Today I learned about metaclasses. 4 Metaclasses are a class'due south course. 5 Meaning every class is an example of a metaclass. 6 The default metaclass is "blazon". 7 8 Classes control features (similar string representations) of all their instances. 9 Metaclasses can control like features for their classes. 10 11 I doubt I'll ever need to make a metaclass, at least not for production code.                      

Summary

Files are lazy iterables, and every bit we loop over a file object, we'll go lines from that file.

When Python reads a file line-past-line, information technology doesn't shop the whole file in memory all at once. Instead, information technology stores a small buffer of upcoming lines in that file, so it's more retentiveness-efficient.

That means looping over files line-by-line is especially important if you're working with really big files.

Topic Trail: Files

Reading from and writing to text files (and sometimes binary files) is an important skill for most Python programmers.

To track your progress on this Python Morsels topic trail, sign in or sign up.

✕

↑

Write more Pythonic code

Need to fill up-in gaps in your Python skills? I send regular emails designed to exercise just that.

searsbothat94.blogspot.com

Source: https://www.pythonmorsels.com/reading-files-line-line/

Share this post