10.3. Alternative File Reading Methods

Once you have a file “object”, the thing returned by the open function, Python provides three methods to read data from that object. The read() method returns the entire contents of the file as a single string (or just some characters if you provide a number as an input parameter. The readlines method returns the entire contents of the entire file as a list of strings, where each item in the list is one line of the file. The readline method reads one line from the file and returns it as a string. The strings returned by readlines or readline will contain the newline character at the end. Table 2 summarizes these methods and the following session shows them in action.

Method Name Use Explanation
write filevar.write(astring) Add astring to the end of the file. filevar must refer to a file that has been opened for writing.
read(n) filevar.read() Reads and returns a string of n characters, or the entire file as a single string if n is not provided.
readline(n) filevar.readline() Returns the next line of the file with all text up to and including the newline character. If n is provided as a parameter than only n characters will be returned if the line is longer than n. Note the parameter n is not supported in the browser version of Python, and in fact is rarely used in practice, you can safely ignore it.
readlines(n) filevar.readlines() Returns a list of strings, each representing a single line of the file. If n is not provided then all lines of the file are returned. If n is provided then n characters are read but n is rounded up so that an entire line is returned. Note Like readline readlines ignores the parameter n in the browser.

In this course, we will generally either iterate through the lines returned by readlines() with a for loop, or use read() to get all of the contents as a single string.

In other programming languages, where they don’t have the convenient for loop method of going through the lines of the file one by one, they use a different pattern which requires a different kind of loop, the while loop. Fortunately, you don’t need to learn this other pattern, and we will put off consideration of while loops until later in this course. We don’t need them for handling data from files.

Note

A common error that novice programmers make is not realizing that all these ways of reading the file contents, use up the file. After you call readlines(), if you call it again you’ll get an empty list.

Check your Understanding

Writing essays for school can be difficult but
many students find that by researching their topic that they
have more to say and are better informed. Here are the university
we require many undergraduate students to take a first year writing requirement
so that they can
have a solid foundation for their writing skills. This comes
in handy for many students.
Different schools have different requirements, but everyone uses
writing at some point in their academic career, be it essays, research papers,
technical write ups, or scripts.
  1. Using the file school_prompt2.txt, find the number of characters in the file and assign that value to the variable num_char.
This summer I will be travelling.
I will go to...
Italy: Rome
Greece: Athens
England: London, Manchester
France: Paris, Nice, Lyon
Spain: Madrid, Barcelona, Granada
Austria: Vienna
I will probably not even want to come back!
However, I wonder how I will get by with all the different languages.
I only know English!
  1. Find the number of lines in the file, travel_plans2.txt, and assign it to the variable num_lines.
Sad upset blue down melancholy somber bitter troubled
Angry mad enraged irate irritable wrathful outraged infuriated
Happy cheerful content elated joyous delighted lively glad
Confused disoriented puzzled perplexed dazed befuddled
Excited eager thrilled delighted
Scared afraid fearful panicked terrified petrified startled
Nervous anxious jittery jumpy tense uneasy apprehensive
  1. Create a string called first_forty that is comprised of the first 40 characters of emotion_words2.txt.
Data file: travel_plans2.txt
This summer I will be travelling.
I will go to...
Italy: Rome
Greece: Athens
England: London, Manchester
France: Paris, Nice, Lyon
Spain: Madrid, Barcelona, Granada
Austria: Vienna
I will probably not even want to come back!
However, I wonder how I will get by with all the different languages.
I only know English!
Data file: school_prompt2.txt
Writing essays for school can be difficult but
many students find that by researching their topic that they
have more to say and are better informed. Here are the university
we require many undergraduate students to take a first year writing requirement
so that they can
have a solid foundation for their writing skills. This comes
in handy for many students.
Different schools have different requirements, but everyone uses
writing at some point in their academic career, be it essays, research papers,
technical write ups, or scripts.
Data file: emotion_words2.txt
Sad upset blue down melancholy somber bitter troubled
Angry mad enraged irate irritable wrathful outraged infuriated
Happy cheerful content elated joyous delighted lively glad
Confused disoriented puzzled perplexed dazed befuddled
Excited eager thrilled delighted
Scared afraid fearful panicked terrified petrified startled
Nervous anxious jittery jumpy tense uneasy apprehensive
Next Section - 10.4. Iterating over lines in a file