# `!csv` --- CSV file reading and writing
csv
**Source code:** `Lib/csv.py`
single: csv pair: data; tabular
------------------------------------------------------------------------
The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. CSV format was used for many years prior to attempts to describe the format in a standardized way in `4180`. The lack of a well-defined standard means that subtle differences often exist in the data produced and consumed by different applications. These differences can make it annoying to process CSV files from multiple sources. Still, while the delimiters and quoting characters vary, the overall format is similar enough that it is possible to write a single module which can efficiently manipulate such data, hiding the details of reading and writing the data from the programmer.
The `!csv` module implements classes to read and write tabular data in CSV format. It allows programmers to say, "write this data in the format preferred by Excel," or "read data from this file which was generated by Excel," without knowing the precise details of the CSV format used by Excel. Programmers can also describe the CSV formats understood by other applications or define their own special-purpose CSV formats.
The `!csv` module's `reader` and `writer` objects read and write sequences. Programmers can also read and write data in dictionary form using the `DictReader` and `DictWriter` classes.
`305` - CSV File API
The Python Enhancement Proposal which proposed this addition to Python.
## Module contents
The `!csv` module defines the following functions:
single: universal newlines; csv.reader function
reader(csvfile, /, dialect='excel', \*\*fmtparams)
Return a `reader object ` that will process lines from the given *csvfile*. A csvfile must be an iterable of strings, each in the reader's defined csv format. A csvfile is most commonly a file-like object or list. If *csvfile* is a file object, it should be opened with `newline=''`.[^1] An optional *dialect* parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the `Dialect` class or one of the strings returned by the `list_dialects` function. The other optional *fmtparams* keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section `csv-fmt-params`.
Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed unless the `QUOTE_NONNUMERIC` format option is specified (in which case unquoted fields are transformed into floats).
A short usage example:
>>> import csv
>>> with open('eggs.csv', newline='') as csvfile:
... spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
... for row in spamreader:
... print(', '.join(row))
Spam, Spam, Spam, Spam, Spam, Baked Beans
Spam, Lovely Spam, Wonderful Spam
where `eggs.csv` contains:
```
Spam Spam Spam Spam Spam |Baked Beans|
Spam |Lovely Spam| |Wonderful Spam|
```
writer(csvfile, /, dialect='excel', \*\*fmtparams)
Return a writer object responsible for converting the user's data into delimited strings on the given file-like object. *csvfile* can be any object with a `~io.TextIOBase.write` method. If *csvfile* is a file object, it should be opened with `newline=''`[^2]. An optional *dialect* parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the `Dialect` class or one of the strings returned by the `list_dialects` function. The other optional *fmtparams* keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about dialects and formatting parameters, see the `csv-fmt-params` section. To make it as easy as possible to interface with modules which implement the DB API, the value `None` is written as the empty string. While this isn't a reversible transformation, it makes it easier to dump SQL NULL data values to CSV files without preprocessing the data returned from a `cursor.fetch*` call. All other non-string data are stringified with `str` before being written.
A short usage example:
import csv
with open('eggs.csv', 'w', newline='') as csvfile:
spamwriter = csv.writer(csvfile, delimiter=' ',
quotechar='|', quoting=csv.QUOTE_MINIMAL)
spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
which writes `eggs.csv` containing:
```
Spam Spam Spam Spam Spam |Baked Beans|
Spam |Lovely Spam| |Wonderful Spam|
```
register_dialect(name, /, dialect='excel', \*\*fmtparams)
Associate *dialect* with *name*. *name* must be a string. The dialect can be specified either by passing a sub-class of `Dialect`, or by *fmtparams* keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about dialects and formatting parameters, see section `csv-fmt-params`.
unregister_dialect(name)
Delete the dialect associated with *name* from the dialect registry. An `Error` is raised if *name* is not a registered dialect name.
get_dialect(name)
Return the dialect associated with *name*. An `Error` is raised if *name* is not a registered dialect name. This function returns an immutable `Dialect`.
list_dialects()
Return the names of all registered dialects.
field_size_limit() field_size_limit(new_limit)
Returns the current maximum field size allowed by the parser. If *new_limit* is given, this becomes the new limit.
The `!csv` module defines the following classes:
Create an object that operates like a regular reader but maps the information in each row to a `dict` whose keys are given by the optional *fieldnames* parameter.
The *fieldnames* parameter is a `sequence`. If *fieldnames* is omitted, the values in the first row of file *f* will be used as the fieldnames and will be omitted from the results. If *fieldnames* is provided, they will be used and the first row will be included in the results. Regardless of how the fieldnames are determined, the dictionary preserves their original ordering.
If a row has more fields than fieldnames, the remaining data is put in a list and stored with the fieldname specified by *restkey* (which defaults to `None`). If a non-blank row has fewer fields than fieldnames, the missing values are filled-in with the value of *restval* (which defaults to `None`).
All other optional or keyword arguments are passed to the underlying `reader` instance.
If the argument passed to *fieldnames* is an iterator, it will be coerced to a `list`.
3.6 Returned rows are now of type `OrderedDict`.
3.8 Returned rows are now of type `dict`.
A short usage example:
>>> import csv
>>> with open('names.csv', newline='') as csvfile:
... reader = csv.DictReader(csvfile)
... for row in reader:
... print(row['first_name'], row['last_name'])
...
Eric Idle
John Cleese
>>> print(row)
{'first_name': 'John', 'last_name': 'Cleese'}
where `names.csv` contains:
```
first_name,last_name
Eric,Idle
John,Cleese
```
The `Dialect` class is a container class whose attributes contain information for how to handle doublequotes, whitespace, delimiters, etc. Due to the lack of a strict CSV specification, different applications produce subtly different CSV data. `Dialect` instances define how `reader` and `writer` instances behave.
All available `Dialect` names are returned by `list_dialects`, and they can be registered with specific `reader` and `writer` classes through their initializer (`__init__`) functions like this:
import csv
with open('students.csv', 'w', newline='') as csvfile:
writer = csv.writer(csvfile, dialect='unix')
The `excel` class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name `'excel'`.
The `excel_tab` class defines the usual properties of an Excel-generated TAB-delimited file. It is registered with the dialect name `'excel-tab'`.
The `unix_dialect` class defines the usual properties of a CSV file generated on UNIX systems, i.e. using `'\n'` as line terminator and quoting all fields. It is registered with the dialect name `'unix'`.
3.2
The `Sniffer` class is used to deduce the format of a CSV file.
The `Sniffer` class provides two methods:
sniff(sample, delimiters=None)
Analyze the given *sample* and return a `Dialect` subclass reflecting the parameters found. If the optional *delimiters* parameter is given, it is interpreted as a string containing possible valid delimiter characters.
The dialect is deduced by parsing the sample with every plausible combination of parameters and choosing the combination which splits the sample into rows with the most consistent number of fields, so the returned dialect is consistent with how `reader` will parse the sample. Raise `Error` if no combination fits the sample, in particular if it is a single column, so there is no delimiter to find.
If several combinations fit the sample equally well --- for example if both `','` and `';'` split every row consistently --- the delimiters listed in the `~Sniffer.preferred` attribute are preferred, in that order, no matter how many times each of them occurs.
The *lineterminator* parameter is deduced separately, by a majority vote among the line endings of the sample. A tie is broken in the order `'\r\n'`, `'\n'`, `'\r'`, so a sample without a complete line gives `'\r\n'`.
3.16 The dialect is now deduced by trial parsing and the results may differ from those of earlier Python versions. The *escapechar* and *lineterminator* parameters can now be detected, and the requested *delimiters* are not restricted to ASCII.
has_header(sample)
Analyze the sample text (presumed to be in CSV format) and return `True` if the first row appears to be a series of column headers. Inspecting each column, one of two key criteria will be considered to estimate if the sample contains a header:
- the second through n-th rows contain numeric values
- the second through n-th rows contain strings where at least one value's length differs from that of the putative header of that column.
Twenty-one rows after the header are sampled; if more than half of the columns + rows meet the criteria, `True` is returned.
> [!NOTE]
> This method is a rough heuristic and may produce both false positives and negatives.
The `Sniffer` class has the following attribute:
preferred
The list of the delimiters preferred for breaking ties, in the order of preference. It can be modified. Its initial value is `[',', '\t', ';', ' ', ':']`.
The `!csv` module defines the following constants:
QUOTE_ALL
Instructs `writer` objects to quote all fields.
QUOTE_MINIMAL
Instructs `writer` objects to only quote those fields which contain special characters such as *delimiter*, *quotechar*, `'\r'`, `'\n'` or any of the characters in *lineterminator*. If *doublequote* is `False` and *escapechar* is set, the *quotechar* is escaped instead of causing the field to be quoted.
QUOTE_NONNUMERIC
Instructs `writer` objects to quote all non-numeric fields.
Instructs `reader` objects to convert all non-quoted fields to type `float`.
> [!NOTE]
> Some numeric types, such as `bool`, `~fractions.Fraction`, or `~enum.IntEnum`, have a string representation that cannot be converted to `float`. They cannot be read in the `QUOTE_NONNUMERIC` and `QUOTE_STRINGS` modes.
QUOTE_NONE
Instructs `writer` objects to never quote fields. When the current *delimiter*, *quotechar*, *escapechar*, `'\r'`, `'\n'` or any of the characters in *lineterminator* occurs in output data it is preceded by the current *escapechar* character. If *escapechar* is not set, the writer will raise `Error` if any characters that require escaping are encountered. Set *quotechar* to `None` to prevent its escaping.
Instructs `reader` objects to perform no special processing of quote characters.
QUOTE_NOTNULL
Instructs `writer` objects to quote all fields which are not `None`. This is similar to `QUOTE_ALL`, except that if a field value is `None` an empty (unquoted) string is written.
Instructs `reader` objects to interpret an empty (unquoted) field as `None` and to otherwise behave as `QUOTE_ALL`.
3.12
QUOTE_STRINGS
Instructs `writer` objects to always place quotes around fields which are strings. This is similar to `QUOTE_NONNUMERIC`, except that if a field value is `None` an empty (unquoted) string is written.
Instructs `reader` objects to interpret an empty (unquoted) string as `None` and to otherwise behave as `QUOTE_NONNUMERIC`.
3.12
Error
Raised by any of the functions when an error is detected.
## Dialects and formatting parameters
To make it easier to specify the format of input and output records, specific formatting parameters are grouped together into dialects. A dialect is a subclass of the `Dialect` class containing various attributes describing the format of the CSV file. When creating `reader` or `writer` objects, the programmer can specify a string or a subclass of the `Dialect` class as the dialect parameter. In addition to, or instead of, the *dialect* parameter, the programmer can also specify individual formatting parameters, which have the same names as the attributes defined below for the `Dialect` class.
Dialects support the following attributes:
Dialect.delimiter
A one-character string used to separate fields. It defaults to `','`.
Dialect.doublequote
Controls how instances of *quotechar* appearing inside a field should themselves be quoted. When `True`, the character is doubled. When `False`, the *escapechar* is used as a prefix to the *quotechar*. It defaults to `True`.
On output, if *doublequote* is `False` and no *escapechar* is set, `Error` is raised if a *quotechar* is found in a field.
Dialect.escapechar
A one-character string used by the writer to escape characters that require escaping:
> - the *delimiter*, the *quotechar*, `'\r'`, `'\n'` and any of the characters in *lineterminator* are escaped if *quoting* is set to `QUOTE_NONE`;
> - the *quotechar* is escaped if *doublequote* is `False`;
> - the *escapechar* itself.
On reading, the *escapechar* removes any special meaning from the following character. It defaults to `None`, which disables escaping.
3.10 Previously the *escapechar* itself was not escaped, which lost it on reading.
3.11 An empty *escapechar* is not allowed.
Dialect.lineterminator
The string used to terminate lines produced by the `writer`. It defaults to `'\r\n'`.
> [!NOTE]
> The `reader` is hard-coded to recognise either `'\r'` or `'\n'` as end-of-line, and ignores *lineterminator*. This behavior may change in the future.
Dialect.quotechar
A one-character string used to quote fields containing special characters, such as the *delimiter* or the *quotechar*, or which contain new-line characters (`'\r'`, `'\n'` or any of the characters in *lineterminator*). It defaults to `'"'`. Can be set to `None` to prevent escaping `'"'` if *quoting* is set to `QUOTE_NONE`.
3.11 An empty *quotechar* is not allowed.
Dialect.quoting
Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the `QUOTE_\* constants ` and defaults to `QUOTE_MINIMAL` if *quotechar* is not `None`, and `QUOTE_NONE` otherwise.
Dialect.skipinitialspace
When `True`, spaces immediately following the *delimiter* are ignored. The default is `False`. When combining `delimiter=' '` with `skipinitialspace=True`, unquoted empty fields are not allowed.
Dialect.strict
When `True`, raise exception `Error` on bad CSV input. The default is `False`.
Dialects support `copy.replace`, which returns a copy of the dialect with the specified formatting parameters replaced.
3.16 Added support for `copy.replace`.
## Reader objects
Reader objects (`DictReader` instances and objects returned by the `reader` function) have the following public methods:
csvreader.\_\_next\_\_()
Return the next row of the reader's iterable object as a list (if the object was returned from `reader`) or a dict (if it is a `DictReader` instance), parsed according to the current `Dialect`. Usually you should call this as `next(reader)`.
Reader objects have the following public attributes:
csvreader.dialect
A read-only description of the dialect in use by the parser.
csvreader.line_num
The number of lines read from the source iterator. This is not the same as the number of records returned, as records can span multiple lines.
DictReader objects have the following public attribute:
DictReader.fieldnames
If not passed as a parameter when creating the object, this attribute is initialized upon first access or when the first record is read from the file.
## Writer objects
`writer` objects (`DictWriter` instances and objects returned by the `writer` function) have the following public methods. A *row* must be an iterable of strings or numbers for `writer` objects and a dictionary mapping fieldnames to strings or numbers (by passing them through `str` first) for `DictWriter` objects. Note that complex numbers are written out surrounded by parens. This may cause some problems for other programs which read CSV files (assuming they support complex numbers at all).
csvwriter.writerow(row, /)
Write the *row* parameter to the writer's file object, formatted according to the current `Dialect`. Return the return value of the call to the *write* method of the underlying file object.
3.5 Added support of arbitrary iterables.
csvwriter.writerows(rows, /)
Write all elements in *rows* (an iterable of *row* objects as described above) to the writer's file object, formatted according to the current dialect.
Writer objects have the following public attribute:
csvwriter.dialect
A read-only description of the dialect in use by the writer.
DictWriter objects have the following public method:
DictWriter.writeheader()
Write a row with the field names (as specified in the constructor) to the writer's file object, formatted according to the current dialect. Return the return value of the `csvwriter.writerow` call used internally.
3.2
3.8 `writeheader` now also returns the value returned by the `csvwriter.writerow` method it uses internally.