Scrittura di file CSV in Python

In questo tutorial impareremo a scrivere file CSV con diversi formati in Python con l'aiuto di esempi.

Useremo esclusivamente il csvmodulo integrato in Python per questa attività. Ma prima, dovremo importare il modulo come:

 import csv

Abbiamo già coperto le basi su come utilizzare il csvmodulo per leggere e scrivere in file CSV. Se non hai idea di come utilizzare il csvmodulo, dai un'occhiata al nostro tutorial su Python CSV: leggere e scrivere file CSV

Utilizzo di base di csv.writer ()

Diamo un'occhiata a un esempio di base di utilizzo csv.writer()per aggiornare le tue conoscenze esistenti.

Esempio 1: scrivi in ​​file CSV con csv.writer ()

Supponiamo di voler scrivere un file CSV con le seguenti voci:

 SN, Name, Contribution 1, Linus Torvalds, Linux Kernel 2, Tim Berners-Lee, World Wide Web 3, Guido van Rossum, Python Programming

Ecco come lo facciamo.

 import csv with open('innovators.csv', 'w', newline='') as file: writer = csv.writer(file) writer.writerow(("SN", "Name", "Contribution")) writer.writerow((1, "Linus Torvalds", "Linux Kernel")) writer.writerow((2, "Tim Berners-Lee", "World Wide Web")) writer.writerow((3, "Guido van Rossum", "Python Programming"))

Quando eseguiamo il programma sopra, viene creato un file innovators.csv nella directory di lavoro corrente con le voci fornite.

Qui abbiamo aperto il file innovators.csv in modalità di scrittura utilizzando open()function.

Per ulteriori informazioni sull'apertura di file in Python, visitare: Input / output file Python

Successivamente, la csv.writer()funzione viene utilizzata per creare un writeroggetto. La writer.writerow()funzione viene quindi utilizzata per scrivere singole righe nel file CSV.

Esempio 2: scrittura di più righe con writerows ()

Se dobbiamo scrivere il contenuto dell'elenco bidimensionale in un file CSV, ecco come possiamo farlo.

 import csv row_list = (("SN", "Name", "Contribution"), (1, "Linus Torvalds", "Linux Kernel"), (2, "Tim Berners-Lee", "World Wide Web"), (3, "Guido van Rossum", "Python Programming")) with open('protagonist.csv', 'w', newline='') as file: writer = csv.writer(file) writer.writerows(row_list)

L'output del programma è lo stesso dell'esempio 1 .

Qui, il nostro elenco bidimensionale viene passato alla writer.writerows()funzione per scrivere il contenuto dell'elenco nel file CSV.

Vediamo ora come scrivere file CSV in diversi formati. Impareremo quindi come personalizzare la csv.writer()funzione per scriverli.

File CSV con delimitatori personalizzati

Per impostazione predefinita, una virgola viene utilizzata come delimitatore in un file CSV. Tuttavia, alcuni file CSV possono utilizzare delimitatori diversi da una virgola. Pochi popolari sono |e .

Supponiamo di voler utilizzare |come delimitatore nel file innovators.csv dell'Esempio 1 . Per scrivere questo file, possiamo passare un delimiterparametro aggiuntivo alla csv.writer()funzione.

Facciamo un esempio.

Esempio 3: scrivere un file CSV con delimitatore di pipe

 import csv data_list = (("SN", "Name", "Contribution"), (1, "Linus Torvalds", "Linux Kernel"), (2, "Tim Berners-Lee", "World Wide Web"), (3, "Guido van Rossum", "Python Programming")) with open('innovators.csv', 'w', newline='') as file: writer = csv.writer(file, delimiter='|') writer.writerows(data_list)

Produzione

 SN | Nome | Contributo 1 | Linus Torvalds | Linux Kernel 2 | Tim Berners-Lee | World Wide Web 3 | Guido van Rossum | Programmazione Python

Come possiamo vedere, il parametro opzionale delimiter = '|'aiuta a specificare l' writeroggetto che il file CSV dovrebbe avere |come delimitatore.

File CSV con citazioni

Alcuni file CSV contengono virgolette attorno a ciascuna o ad alcune voci.

Prendiamo quotes.csv come esempio, con le seguenti voci:

 "SN"; "Nome"; "Citazioni" 1; "Buddha"; "Cosa pensiamo di diventare" 2; "Mark Twain"; "Non rimpiangere mai nulla che ti abbia fatto sorridere" 3; "Oscar Wilde"; "Sii te stesso tutti gli altri sono già stati presi "

L'uso csv.writer()predefinito non aggiungerà queste virgolette alle voci.

Per aggiungerli, dovremo utilizzare un altro parametro opzionale chiamato quoting.

Facciamo un esempio di come le virgolette possono essere utilizzate attorno ai valori non numerici e ;come delimitatori.

Esempio 4: scrivi file CSV tra virgolette

 import csv row_list = ( ("SN", "Name", "Quotes"), (1, "Buddha", "What we think we become"), (2, "Mark Twain", "Never regret anything that made you smile"), (3, "Oscar Wilde", "Be yourself everyone else is already taken") ) with open('quotes.csv', 'w', newline='') as file: writer = csv.writer(file, quoting=csv.QUOTE_NONNUMERIC, delimiter=';') writer.writerows(row_list)

Produzione 

 "SN"; "Nome"; "Citazioni" 1; "Buddha"; "Cosa pensiamo di diventare" 2; "Mark Twain"; "Non rimpiangere mai nulla che ti abbia fatto sorridere" 3; "Oscar Wilde"; "Sii te stesso tutti gli altri sono già stati presi "

Qui, il file quotes.csv viene creato nella directory di lavoro con le voci precedenti.

Come puoi vedere, siamo passati csv.QUOTE_NONNUMERICal quotingparametro. È una costante definita dal csvmodulo.

csv.QUOTE_NONNUMERICspecifica l' writeroggetto che le virgolette dovrebbero essere aggiunte intorno alle voci non numeriche.

Ci sono altre 3 costanti predefinite che puoi passare al quotingparametro:

  • csv.QUOTE_ALL- Specifica l' writeroggetto per scrivere il file CSV con virgolette attorno a tutte le voci.
  • csv.QUOTE_MINIMAL- Specifica l' writeroggetto per citare solo quei campi che contengono caratteri speciali ( delimitatore , quotechar o qualsiasi carattere in lineterminator )
  • csv.QUOTE_NONE- Specifica l' writeroggetto che nessuna delle voci deve essere citata. È il valore predefinito.

File CSV con virgolette personalizzate

We can also write CSV files with custom quoting characters. For that, we will have to use an optional parameter called quotechar.

Let's take an example of writing quotes.csv file in Example 4, but with * as the quoting character.

Example 5: Writing CSV files with custom quoting character

 import csv row_list = ( ("SN", "Name", "Quotes"), (1, "Buddha", "What we think we become"), (2, "Mark Twain", "Never regret anything that made you smile"), (3, "Oscar Wilde", "Be yourself everyone else is already taken") ) with open('quotes.csv', 'w', newline='') as file: writer = csv.writer(file, quoting=csv.QUOTE_NONNUMERIC, delimiter=';', quotechar='*') writer.writerows(row_list)

Output 

 *SN*;*Name*;*Quotes* 1;*Buddha*;*What we think we become* 2;*Mark Twain*;*Never regret anything that made you smile* 3;*Oscar Wilde*;*Be yourself everyone else is already taken*

Here, we can see that quotechar='*' parameter instructs the writer object to use * as quote for all non-numeric values.

Dialects in CSV module

Notice in Example 5 that we have passed multiple parameters (quoting, delimiter and quotechar) to the csv.writer() function.

This practice is acceptable when dealing with one or two files. But it will make the code more redundant and ugly once we start working with multiple CSV files with similar formats.

As a solution to this, the csv module offers dialect as an optional parameter.

Dialect helps in grouping together many specific formatting patterns like delimiter, skipinitialspace, quoting, escapechar into a single dialect name.

It can then be passed as a parameter to multiple writer or reader instances.

Example 6: Write CSV file using dialect

Suppose we want to write a CSV file (office.csv) with the following content:

 "ID"|"Name"|"Email" "A878"|"Alfonso K. Hamby"|"[email protected]" "F854"|"Susanne Briard"|"[email protected]" "E833"|"Katja Mauer"|"[email protected]"

The CSV file has quotes around each entry and uses | as a delimiter.

Instead of passing two individual formatting patterns, let's look at how to use dialects to write this file.

 import csv row_list = ( ("ID", "Name", "Email"), ("A878", "Alfonso K. Hamby", "[email protected]"), ("F854", "Susanne Briard", "[email protected]"), ("E833", "Katja Mauer", "[email protected]") ) csv.register_dialect('myDialect', delimiter='|', quoting=csv.QUOTE_ALL) with open('office.csv', 'w', newline='') as file: writer = csv.writer(file, dialect='myDialect') writer.writerows(row_list)

Output 

 "ID"|"Name"|"Email" "A878"|"Alfonso K. Hamby"|"[email protected]" "F854"|"Susanne Briard"|"[email protected]" "E833"|"Katja Mauer"|"[email protected]"

Here, office.csv is created in the working directory with the above contents.

From this example, we can see that the csv.register_dialect() function is used to define a custom dialect. Its syntax is:

 csv.register_dialect(name(, dialect(, **fmtparams)))

The custom dialect requires a name in the form of a string. Other specifications can be done either by passing a sub-class of the Dialect class, or by individual formatting patterns as shown in the example.

While creating the writer object, we pass dialect='myDialect' to specify that the writer instance must use that particular dialect.

The advantage of using dialect is that it makes the program more modular. Notice that we can reuse myDialect to write other CSV files without having to re-specify the CSV format.

Write CSV files with csv.DictWriter()

The objects of csv.DictWriter() class can be used to write to a CSV file from a Python dictionary.

The minimal syntax of the csv.DictWriter() class is:

 csv.DictWriter(file, fieldnames)

Here,

  • file - CSV file where we want to write to
  • fieldnames - a list object which should contain the column headers specifying the order in which data should be written in the CSV file

Example 7: Python csv.DictWriter()

 import csv with open('players.csv', 'w', newline='') as file: fieldnames = ('player_name', 'fide_rating') writer = csv.DictWriter(file, fieldnames=fieldnames) writer.writeheader() writer.writerow(('player_name': 'Magnus Carlsen', 'fide_rating': 2870)) writer.writerow(('player_name': 'Fabiano Caruana', 'fide_rating': 2822)) writer.writerow(('player_name': 'Ding Liren', 'fide_rating': 2801))

Output

The program creates a players.csv file with the following entries:

 player_name,fide_rating Magnus Carlsen,2870 Fabiano Caruana,2822 Ding Liren,2801

The full syntax of the csv.DictWriter() class is:

 csv.DictWriter(f, fieldnames, restval='', extrasaction='raise', dialect='excel', *args, **kwds)

To learn more about it in detail, visit: Python csv.DictWriter() class

CSV files with lineterminator

A lineterminator is a string used to terminate lines produced by writer objects. The default value is . You can change its value by passing any string as a lineterminator parameter.

However, the reader object only recognizes or as lineterminator values. So using other characters as line terminators is highly discouraged.

doublequote & escapechar in CSV module

In order to separate delimiter characters in the entries, the csv module by default quotes the entries using quotation marks.

So, if you had an entry: He is a strong, healthy man, it will be written as: "He is a strong, healthy man".

Similarly, the csv module uses double quotes in order to escape the quote character present in the entries by default.

If you had an entry: Go to "programiz.com", it would be written as: "Go to ""programiz.com""".

Here, we can see that each " is followed by a " to escape the previous one.

doublequote

It handles how quotechar present in the entry themselves are quoted. When True, the quoting character is doubled and when False, the escapechar is used as a prefix to the quotechar. By default its value is True.

escapechar

escapechar parameter is a string to escape the delimiter if quoting is set to csv.QUOTE_NONE and quotechar if doublequote is False. Its default value is None.

Example 8: Using escapechar in csv writer

 import csv row_list = ( ('Book', 'Quote'), ('Lord of the Rings', '"All we have to decide is what to do with the time that is given us."'), ('Harry Potter', '"It matters not what someone is born, but what they grow to be."') ) with open('book.csv', 'w', newline='') as file: writer = csv.writer(file, escapechar='/', quoting=csv.QUOTE_NONE) writer.writerows(row_list)

Output 

 Book,Quote Lord of the Rings,/"All we have to decide is what to do with the time that is given us./" Harry Potter,/"It matters not what someone is born/, but what they grow to be./"

Here, we can see that / is prefix to all the " and , because we specified quoting=csv.QUOTE_NONE.

If it wasn't defined, then, the output would be:

 Book,Quote Lord of the Rings,"""All we have to decide is what to do with the time that is given us.""" Harry Potter,"""It matters not what someone is born, but what they grow to be."""

Since we allow quoting, the entries with special characters(" in this case) are double-quoted. The entries with delimiter are also enclosed within quote characters.(Starting and closing quote characters)

The remaining quote characters are to escape the actual " present as part of the string, so that they are not interpreted as quotechar.

Note: The csv module can also be used for other file extensions (like: .txt) as long as their contents are in proper structure.

Lettura consigliata: leggi i file CSV in Python

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