Python skriv til fil - Åbn, læs, tilføj og andre filhåndteringsfunktioner forklaret

Velkommen

Hej! Hvis du vil lære at arbejde med filer i Python, er denne artikel noget for dig. Arbejde med filer er en vigtig færdighed, som enhver Python-udvikler skal lære, så lad os komme i gang.

I denne artikel lærer du:

  • Sådan åbnes en fil.
  • Sådan læses en fil.
  • Sådan oprettes en fil.
  • Sådan ændres en fil.
  • Sådan lukkes en fil.
  • Sådan åbnes filer til flere operationer.
  • Sådan arbejder du med filobjektmetoder.
  • Sådan slettes filer.
  • Sådan arbejder du med kontekstledere, og hvorfor de er nyttige.
  • Sådan håndteres undtagelser, der kan hæves, når du arbejder med filer.
  • og mere!

Lad os begynde! ✨

? Arbejde med filer: Grundlæggende syntaks

En af de mest vigtige funktioner, som du skal bruge, når du arbejder med filer i Python er open(), en indbygget funktion, der åbner en fil og giver dit program til at bruge det og arbejde med det.

Dette er den grundlæggende syntaks :

? Tip: Det er de to mest anvendte argumenter for at kalde denne funktion. Der er yderligere seks valgfrie argumenter. For at lære mere om dem, skal du læse denne artikel i dokumentationen.

Første parameter: Fil

Den første parameter for open()funktionen er fileden absolutte eller relative sti til den fil, du prøver at arbejde med.

Vi bruger normalt en relativ sti, der angiver, hvor filen er placeret i forhold til placeringen af ​​scriptet (Python-fil), der kalder open()funktionen.

For eksempel kalder stien i denne funktion:

open("names.txt") # The relative path is "names.txt"

Indeholder kun filens navn. Dette kan bruges, når den fil, du prøver at åbne, er i samme bibliotek eller mappe som Python-scriptet, som denne:

Men hvis filen er i en indlejret mappe, som denne:

Derefter skal vi bruge en bestemt sti til at fortælle funktionen, at filen er i en anden mappe.

I dette eksempel ville dette være stien:

open("data/names.txt")

Bemærk, at vi data/først skriver (navnet på mappen efterfulgt af a /) og derefter names.txt(navnet på filen med filtypenavnet).

? Tip: De tre bogstaver, .txtder følger prikken i, names.txter filens "udvidelse" eller dens type. I dette tilfælde .txtangiver det, at det er en tekstfil.

Anden parameter: Mode

Den anden parameter for open()funktionen er mode, en streng med et tegn. Denne enkelt karakter fortæller dybest set Python, hvad du planlægger at gøre med filen i dit program.

Tilgængelige tilstande er:

  • Læs ( "r").
  • Tilføj ( "a")
  • Skriv ( "w")
  • Opret ( "x")

Du kan også vælge at åbne filen i:

  • Teksttilstand ( "t")
  • Binær tilstand ( "b")

For at bruge tekst eller binær tilstand skal du tilføje disse tegn til hovedtilstanden. For eksempel: "wb"betyder skrivning i binær tilstand.

? Tip: Standardtilstandene læses ( "r") og tekst ( "t"), hvilket betyder "åben for læsning af tekst" ( "rt"), så du behøver ikke at angive dem, open()hvis du vil bruge dem, fordi de er tildelt som standard. Du kan bare skrive open().

Hvorfor tilstande?

Det giver virkelig mening for Python kun at give visse tilladelser baseret på hvad du planlægger at gøre med filen, ikke? Hvorfor skal Python tillade, at dit program gør mere end nødvendigt? Dette er grundlæggende hvorfor tilstande findes.

Tænk over det - at lade et program gøre mere end nødvendigt kan være problematisk. For eksempel, hvis du kun har brug for at læse indholdet af en fil, kan det være farligt at tillade dit program at ændre det uventet, hvilket potentielt kan introducere fejl.

? Sådan læses en fil

Nu hvor du ved mere om de argumenter, som open()funktionen tager, lad os se, hvordan du kan åbne en fil og gemme den i en variabel for at bruge den i dit program.

Dette er den grundlæggende syntaks:

Vi tildeler simpelthen den værdi, der returneres til en variabel. For eksempel:

names_file = open("data/names.txt", "r")

Jeg ved, at du måske spørger: hvilken type værdi returneres af open()?

Tja, en fil objekt .

Lad os tale lidt om dem.

Filobjekter

Ifølge Python-dokumentationen er et filobjekt :

Et objekt, der udsætter en filorienteret API (med metoder som read () eller skriv ()) for en underliggende ressource.

Dette fortæller os grundlæggende, at et filobjekt er et objekt, der lader os arbejde og interagere med eksisterende filer i vores Python-program.

Filobjekter har attributter, såsom:

  • name: the name of the file.
  • closed: True if the file is closed. False otherwise.
  • mode: the mode used to open the file.

For example:

f = open("data/names.txt", "a") print(f.mode) # Output: "a"

Now let's see how you can access the content of a file through a file object.

Methods to Read a File

For us to be able to work file objects, we need to have a way to "interact" with them in our program and that is exactly what methods do. Let's see some of them.

Read()

The first method that you need to learn about is read(),which returns the entire content of the file as a string.

Here we have an example:

f = open("data/names.txt") print(f.read())

The output is:

Nora Gino Timmy William

You can use the type() function to confirm that the value returned by f.read() is a string:

print(type(f.read())) # Output 

Yes, it's a string!

In this case, the entire file was printed because we did not specify a maximum number of bytes, but we can do this as well.

Here we have an example:

f = open("data/names.txt") print(f.read(3))

The value returned is limited to this number of bytes:

Nor

❗️Important: You need to close a file after the task has been completed to free the resources associated to the file. To do this, you need to call the close() method, like this:

Readline() vs. Readlines()

You can read a file line by line with these two methods. They are slightly different, so let's see them in detail.

readline() reads one line of the file until it reaches the end of that line. A trailing newline character (\n) is kept in the string.

? Tip: Optionally, you can pass the size, the maximum number of characters that you want to include in the resulting string.

For example:

f = open("data/names.txt") print(f.readline()) f.close()

The output is:

Nora 

This is the first line of the file.

In contrast, readlines() returns a list with all the lines of the file as individual elements (strings). This is the syntax:

For example:

f = open("data/names.txt") print(f.readlines()) f.close()

The output is:

['Nora\n', 'Gino\n', 'Timmy\n', 'William']

Notice that there is a \n (newline character) at the end of each string, except the last one.

? Tip: You can get the same list with list(f).

You can work with this list in your program by assigning it to a variable or using it in a loop:

f = open("data/names.txt") for line in f.readlines(): # Do something with each line f.close()

We can also iterate over f directly (the file object) in a loop:

f = open("data/names.txt", "r") for line in f: # Do something with each line f.close()

Those are the main methods used to read file objects. Now let's see how you can create files.

? Sådan oprettes en fil

Hvis du har brug for at oprette en fil "dynamisk" ved hjælp af Python, kan du gøre det med "x"tilstanden.

Lad os se hvordan. Dette er den grundlæggende syntaks:

Her er et eksempel. Dette er min nuværende arbejdsmappe:

Hvis jeg kører denne kodelinje:

f = open("new_file.txt", "x")

En ny fil med det navn oprettes:

Med denne tilstand kan du oprette en fil og derefter skrive til den dynamisk ved hjælp af metoder, som du vil lære på få øjeblikke.

? Tip: Filen vil være i starten tom, indtil du ændrer den.

En nysgerrig ting er, at hvis du forsøger at køre denne linje igen, og der allerede findes en fil med det navn, vil du se denne fejl:

Traceback (most recent call last): File "", line 8, in  f = open("new_file.txt", "x") FileExistsError: [Errno 17] File exists: 'new_file.txt'

Ifølge Python-dokumentationen er denne undtagelse (runtime error):

Hæves, når du prøver at oprette en fil eller mappe, der allerede findes.

Now that you know how to create a file, let's see how you can modify it.

? How to Modify a File

To modify (write to) a file, you need to use the write() method. You have two ways to do it (append or write) based on the mode that you choose to open it with. Let's see them in detail.

Append

"Appending" means adding something to the end of another thing. The "a" mode allows you to open a file to append some content to it.

For example, if we have this file:

And we want to add a new line to it, we can open it using the "a" mode (append) and then, call the write() method, passing the content that we want to append as argument.

This is the basic syntax to call the write()method:

Here's an example:

f = open("data/names.txt", "a") f.write("\nNew Line") f.close()

? Tip: Notice that I'm adding \n before the line to indicate that I want the new line to appear as a separate line, not as a continuation of the existing line.

This is the file now, after running the script:

? Tip: The new line might not be displayed in the file untilf.close() runs.

Write

Sometimes, you may want to delete the content of a file and replace it entirely with new content. You can do this with the write() method if you open the file with the "w" mode.

Here we have this text file:

If I run this script:

f = open("data/names.txt", "w") f.write("New Content") f.close() 

This is the result:

As you can see, opening a file with the "w" mode and then writing to it replaces the existing content.

? Tip: The write() method returns the number of characters written.

If you want to write several lines at once, you can use the writelines() method, which takes a list of strings. Each string represents a line to be added to the file.

Here's an example. This is the initial file:

If we run this script:

f = open("data/names.txt", "a") f.writelines(["\nline1", "\nline2", "\nline3"]) f.close()

The lines are added to the end of the file:

Open File For Multiple Operations

Now you know how to create, read, and write to a file, but what if you want to do more than one thing in the same program? Let's see what happens if we try to do this with the modes that you have learned so far:

If you open a file in "r" mode (read), and then try to write to it:

f = open("data/names.txt") f.write("New Content") # Trying to write f.close()

You will get this error:

Traceback (most recent call last): File "", line 9, in  f.write("New Content") io.UnsupportedOperation: not writable

Similarly, if you open a file in "w" mode (write), and then try to read it:

f = open("data/names.txt", "w") print(f.readlines()) # Trying to read f.write("New Content") f.close()

You will see this error:

Traceback (most recent call last): File "", line 14, in  print(f.readlines()) io.UnsupportedOperation: not readable

The same will occur with the "a" (append) mode.

How can we solve this? To be able to read a file and perform another operation in the same program, you need to add the "+" symbol to the mode, like this:

f = open("data/names.txt", "w+") # Read + Write
f = open("data/names.txt", "a+") # Read + Append
f = open("data/names.txt", "r+") # Read + Write

Very useful, right? This is probably what you will use in your programs, but be sure to include only the modes that you need to avoid potential bugs.

Sometimes files are no longer needed. Let's see how you can delete files using Python.

? How to Delete Files

To remove a file using Python, you need to import a module called os which contains functions that interact with your operating system.

? Tip: A module is a Python file with related variables, functions, and classes.

Particularly, you need the remove()function. This function takes the path to the file as argument and deletes the file automatically.

Let's see an example. We want to remove the file called sample_file.txt.

To do it, we write this code:

import os os.remove("sample_file.txt")
  • The first line: import os is called an "import statement". This statement is written at the top of your file and it gives you access to the functions defined in the os module.
  • The second line: os.remove("sample_file.txt") removes the file specified.

? Tip: you can use an absolute or a relative path.

Now that you know how to delete files, let's see an interesting tool... Context Managers!

? Meet Context Managers

Context Managers are Python constructs that will make your life much easier. By using them, you don't need to remember to close a file at the end of your program and you have access to the file in the particular part of the program that you choose.

Syntax

This is an example of a context manager used to work with files:

? Tip: The body of the context manager has to be indented, just like we indent loops, functions, and classes. If the code is not indented, it will not be considered part of the context manager.

When the body of the context manager has been completed, the file closes automatically.

with open("", "") as : # Working with the file... # The file is closed here!

Example

Here's an example:

with open("data/names.txt", "r+") as f: print(f.readlines()) 

This context manager opens the names.txt file for read/write operations and assigns that file object to the variable f. This variable is used in the body of the context manager to refer to the file object.

Trying to Read it Again

After the body has been completed, the file is automatically closed, so it can't be read without opening it again. But wait! We have a line that tries to read it again, right here below:

with open("data/names.txt", "r+") as f: print(f.readlines()) print(f.readlines()) # Trying to read the file again, outside of the context manager

Lad os se hvad der sker:

Traceback (most recent call last): File "", line 21, in  print(f.readlines()) ValueError: I/O operation on closed file.

Denne fejl smides, fordi vi prøver at læse en lukket fil. Fantastisk, ikke? Context manager udfører alt det tunge arbejde for os, det er læsbart og kortfattet.

? Sådan håndteres undtagelser, når du arbejder med filer

Når du arbejder med filer, kan der opstå fejl. Nogle gange har du muligvis ikke de nødvendige tilladelser til at ændre eller få adgang til en fil, ellers findes der muligvis ikke en fil.

Som programmør skal du forudse disse omstændigheder og håndtere dem i dit program for at undgå pludselige nedbrud, der helt sikkert kan påvirke brugeroplevelsen.

Lad os se nogle af de mest almindelige undtagelser (runtime-fejl), som du måske finder, når du arbejder med filer:

FileNotFoundError

Ifølge Python-dokumentationen er denne undtagelse:

Hæves, når der anmodes om en fil eller mappe, men ikke findes.

For example, if the file that you're trying to open doesn't exist in your current working directory:

f = open("names.txt")

You will see this error:

Traceback (most recent call last): File "", line 8, in  f = open("names.txt") FileNotFoundError: [Errno 2] No such file or directory: 'names.txt'

Let's break this error down this line by line:

  • File "", line 8, in . This line tells you that the error was raised when the code on the file located in was running. Specifically, when line 8 was executed in .
  • f = open("names.txt"). This is the line that caused the error.
  • FileNotFoundError: [Errno 2] No such file or directory: 'names.txt' . This line says that a FileNotFoundError exception was raised because the file or directory names.txt doesn't exist.

? Tip: Python is very descriptive with the error messages, right? This is a huge advantage during the process of debugging.

PermissionError

This is another common exception when working with files. According to the Python Documentation, this exception is:

Oprejst, når du prøver at køre en operation uden de tilstrækkelige adgangsrettigheder - for eksempel filsystemtilladelser.

Denne undtagelse hæves, når du prøver at læse eller ændre en fil, der ikke har adgangstilladelse. Hvis du prøver at gøre det, vil du se denne fejl:

Traceback (most recent call last): File "", line 8, in  f = open("") PermissionError: [Errno 13] Permission denied: 'data'

IsADirectoryError

Ifølge Python-dokumentationen er denne undtagelse:

Hæves, når der kræves en filhandling i et bibliotek.

Denne særlige undtagelse hæves, når du prøver at åbne eller arbejde på en mappe i stedet for en fil, så vær virkelig forsigtig med den sti, du passerer som argument.

Sådan håndteres undtagelser

For at håndtere disse undtagelser kan du bruge en prøve / undtagen erklæring. Med denne erklæring kan du "fortælle" dit program, hvad du skal gøre, hvis der sker noget uventet.

Dette er den grundlæggende syntaks:

try: # Try to run this code except : # If an exception of this type is raised, stop the process and jump to this block 

Here you can see an example with FileNotFoundError:

try: f = open("names.txt") except FileNotFoundError: print("The file doesn't exist")

This basically says:

  • Try to open the file names.txt.
  • If a FileNotFoundError is thrown, don't crash! Simply print a descriptive statement for the user.

? Tip: You can choose how to handle the situation by writing the appropriate code in the except block. Perhaps you could create a new file if it doesn't exist already.

To close the file automatically after the task (regardless of whether an exception was raised or not in the try block) you can add the finally block.

try: # Try to run this code except : # If this exception is raised, stop the process immediately and jump to this block finally: # Do this after running the code, even if an exception was raised

This is an example:

try: f = open("names.txt") except FileNotFoundError: print("The file doesn't exist") finally: f.close()

There are many ways to customize the try/except/finally statement and you can even add an else block to run a block of code only if no exceptions were raised in the try block.

? Tip: To learn more about exception handling in Python, you may like to read my article: "How to Handle Exceptions in Python: A Detailed Visual Introduction".

? In Summary

  • You can create, read, write, and delete files using Python.
  • File objects have their own set of methods that you can use to work with them in your program.
  • Context Managers help you work with files and manage them by closing them automatically when a task has been completed.
  • Exception handling is key in Python. Common exceptions when you are working with files include FileNotFoundError, PermissionError and IsADirectoryError. They can be handled using try/except/else/finally.

Jeg håber virkelig, du kunne lide min artikel og fandt det nyttigt. Nu kan du arbejde med filer i dine Python-projekter. Tjek mine online kurser. Følg mig på Twitter. ⭐️