Pythoni faili kirjutamine - lahti seletamine, lugemine, lisamine ja muud failikäsitlusfunktsioonid

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Tere! Kui soovite õppida Pythonis failidega töötamist, siis see artikkel on mõeldud teile. Failidega töötamine on oluline oskus, mida iga Pythoni arendaja peaks õppima, nii et alustame.

Selles artiklis saate teada:

  • Kuidas faili avada.
  • Kuidas faili lugeda.
  • Kuidas faili luua.
  • Kuidas faili muuta.
  • Kuidas fail sulgeda.
  • Failide avamine mitme toimingu jaoks.
  • Kuidas toimida faili objektimeetoditega.
  • Kuidas faile kustutada.
  • Kuidas kontekstihalduritega töötada ja miks need on kasulikud.
  • Kuidas käituda failidega töötamisel tekkida võivate eranditega?
  • ja veel!

Alustagem! ✨

? Failidega töötamine: põhisüntaks

Üks olulisemaid funktsioone, mida peate Pythonis failidega töötades kasutama open(), on sisseehitatud funktsioon, mis avab faili ja võimaldab teie programmil seda kasutada ja sellega töötada.

See on põhiline süntaks :

? Nõuanne. Need on kaks kõige sagedamini kasutatavat argumenti selle funktsiooni kutsumiseks. On veel kuus valikulist argumenti. Nende kohta lisateabe saamiseks lugege seda artiklit dokumentatsioonist.

Esimene parameeter: fail

Funktsiooni esimene parameeter open()on fileabsoluutne või suhteline tee faili juurde, millega proovite töötada.

Tavaliselt kasutame suhtelist teed, mis näitab faili asukohta open()funktsiooni kutsuva skripti (Pythoni fail) asukoha suhtes .

Näiteks selle funktsiooni tee kutsub:

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

Sisaldab ainult faili nime. Seda saab kasutada siis, kui avatav fail on Pythoni skriptiga samas kataloogis või kaustas, näiteks:

Kuid kui fail asub pesastatud kaustas, toimige järgmiselt.

Seejärel peame funktsioonile teatama, et fail asub teises kaustas, kasutades konkreetset teed.

Selles näites oleks see tee:

open("data/names.txt")

Pange tähele, et me kirjutame data/kõigepealt (kausta nimi, millele järgneb a /) ja seejärel names.txt(laiendiga faili nimi).

? Nõuanne . Kolm .txtpunkti, mis järgnevad punktile names.txt, on faili "laiend" või selle tüüp. Sel juhul .txtnäitab see , et see on tekstifail.

Teine parameeter: režiim

Funktsiooni teine ​​parameeter open()on modeühe märgiga string. See üks märk ütleb Pythonile põhimõtteliselt, mida kavatsete oma programmi failiga teha.

Saadaval on järgmised režiimid:

  • Loe ( "r").
  • Lisa ( "a")
  • Kirjuta ( "w")
  • Loo ( "x")

Võite ka faili avada järgmises režiimis:

  • Tekstirežiim ( "t")
  • Binaarrežiim ( "b")

Teksti- või kahendrežiimi kasutamiseks peate need märgid põhirežiimi lisama. Näiteks: "wb"tähendab kirjutamist binaarses režiimis.

? Näpunäide: Vaikerežiimid on read ( "r") ja text ( "t"), mis tähendab "avatud teksti lugemiseks" ( "rt"), nii et open()kui soovite neid kasutada, ei pea te neid määrama, kuna need on vaikimisi määratud. Võite lihtsalt kirjutada open().

Miks režiimid?

Pythoni jaoks on tõesti mõistlik anda ainult teatud õigused, lähtudes sellest, mida plaanite failiga teha, eks? Miks peaks Python lubama teie programmil teha rohkem kui vaja? Põhimõtteliselt on seetõttu režiimid olemas.

Mõelge sellele - kui lubate programmil teha rohkem kui vaja, võib see olla problemaatiline. Näiteks kui peate lugema ainult faili sisu, võib olla ohtlik lubada teie programmil seda ootamatult muuta, mis võib põhjustada vigu.

? Kuidas faili lugeda

Nüüd, kui teate rohkem open()funktsiooni kasutatavate argumentide kohta , vaatame, kuidas saate faili avada ja selle muutujasse salvestada, et seda oma programmis kasutada.

See on põhiline süntaks:

Määrame lihtsalt muutujale tagastatud väärtuse. Näiteks:

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

Ma tean, et võite küsida: mis tüüpi väärtuse tagastab open()?

Noh, faili objekti .

Räägime neist natuke.

Faili objektid

Vastavalt Pythoni dokumentatsioonile on failiobjekt :

Objekt, mis paljastab failipõhise API (koos selliste meetoditega nagu read () või write ()) aluseks olevale ressursile.

Põhimõtteliselt ütleb see meile, et failiobjekt on objekt, mis võimaldab meil töötada ja suhelda olemasolevate failidega meie Pythoni programmis.

Failiobjektidel on atribuudid, näiteks:

  • 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.

? Kuidas faili luua

Kui peate Pythoni abil "dünaamiliselt" faili looma, saate seda teha "x"režiimiga.

Vaatame kuidas. See on põhiline süntaks:

Siin on näide. See on minu praegune töökataloog:

Kui käivitan selle koodirea:

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

Luuakse uus selle nimega fail:

Selles režiimis saate luua faili ja seejärel sellele dünaamiliselt kirjutada, kasutades meetodeid, mida õpite mõne hetkega.

? Nõuanne: fail on esialgu tühi, kuni seda muudate.

Kummaline on see, et kui proovite seda rida uuesti käivitada ja selle nimega fail on juba olemas, näete seda viga:

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

Vastavalt Pythoni dokumentatsioonile on see erand (käitustõrge):

Tõstetakse üles juba olemasoleva faili või kataloogi loomisel.

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

Vaatame mis juhtub:

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

See viga visati, kuna proovime lugeda suletud faili. Äge, eks? Kontekstihaldur teeb meie jaoks kogu raske töö ära, see on loetav ja kokkuvõtlik.

? Kuidas toimida failidega töötamisel eranditega

Failidega töötamisel võivad ilmneda vead. Mõnikord ei pruugi teil olla faili muutmiseks või sellele juurdepääsu saamiseks vajalikke õigusi või faili ei pruugi isegi olla.

Programmeerijana peate neid asjaolusid ette nägema ja oma programmis käsitsema, et vältida ootamatuid krahhe, mis võivad kindlasti kasutajakogemust mõjutada.

Vaatame mõningaid levinumaid erandeid (käitamisvead), mis võivad failidega töötamisel leida:

FileNotFoundError

Vastavalt Pythoni dokumentatsioonile on see erand:

Tõstetakse, kui faili või kataloogi taotletakse, kuid seda pole olemas.

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:

Tõstetakse üles, kui proovite käivitada toimingut ilma piisavate juurdepääsuõigusteta - näiteks failisüsteemi lubadeta.

See erand ilmneb siis, kui proovite lugeda või muuta faili, millel puudub juurdepääsuluba. Kui proovite seda teha, näete järgmist viga:

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

IsADirectoryError

Vastavalt Pythoni dokumentatsioonile on see erand:

Tõstetakse, kui kataloogis soovitakse failitoimingut.

See konkreetne erand ilmneb siis, kui proovite faili asemel kataloogi avada või sellega töötada, seega olge argumendina edastatud teega väga ettevaatlik.

Kuidas käsitleda erandeid

Nende erandite käsitlemiseks võite kasutada käsku try / paitsi. Selle väitega saate oma programmile "öelda", mida teha, kui juhtub midagi ootamatut.

See on põhiline süntaks:

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.

Loodan väga, et teile meeldis minu artikkel ja see oli teile kasulik. Nüüd saate Pythoni projektides failidega töötada. Vaadake minu veebikursusi. Jälgi mind Twitteris. ⭐️