class CSV

This class provides a complete interface to CSV files and data. It offers tools to enable you to read and write to and from Strings or IO objects, as needed.

The most generic interface of the library is:

csv = CSV.new(io, **options)

# Reading: IO object should be open for read
csv.read # => array of rows
# or
csv.each do |row|
  # ...
end
# or
row = csv.shift

# Writing: IO object should be open for write
csv << row

There are several specialized class methods for one-statement reading or writing, described in the Specialized Methods section.

If a String is passed into ::new, it is internally wrapped into a StringIO object.

options can be used for specifying the particular CSV flavor (column separators, row separators, value quoting and so on), and for data conversion, see Data Conversion section for the description of the latter.

Specialized Methods¶ ↑

Reading¶ ↑

# From a file: all at once
arr_of_rows = CSV.read("path/to/file.csv", **options)
# iterator-style:
CSV.foreach("path/to/file.csv", **options) do |row|
  # ...
end

# From a string
arr_of_rows = CSV.parse("CSV,data,String", **options)
# or
CSV.parse("CSV,data,String", **options) do |row|
  # ...
end

Writing¶ ↑

# To a file
CSV.open("path/to/file.csv", "wb") do |csv|
  csv << ["row", "of", "CSV", "data"]
  csv << ["another", "row"]
  # ...
end

# To a String
csv_string = CSV.generate do |csv|
  csv << ["row", "of", "CSV", "data"]
  csv << ["another", "row"]
  # ...
end

Shortcuts¶ ↑

# Core extensions for converting one line
csv_string = ["CSV", "data"].to_csv   # to CSV
csv_array  = "CSV,String".parse_csv   # from CSV

# CSV() method
CSV             { |csv_out| csv_out << %w{my data here} }  # to $stdout
CSV(csv = "")   { |csv_str| csv_str << %w{my data here} }  # to a String
CSV($stderr)    { |csv_err| csv_err << %w{my data here} }  # to $stderr
CSV($stdin)     { |csv_in|  csv_in.each { |row| p row } }  # from $stdin

Options¶ ↑

The default values for options are:

DEFAULT_OPTIONS = {
  # For both parsing and generating.
  col_sep:            ",",
  row_sep:            :auto,
  quote_char:         '"',
  # For parsing.
  field_size_limit:   nil,
  converters:         nil,
  unconverted_fields: nil,
  headers:            false,
  return_headers:     false,
  header_converters:  nil,
  skip_blanks:        false,
  skip_lines:         nil,
  liberal_parsing:    false,
  nil_value:          nil,
  empty_value:        "",
  # For generating.
  write_headers:      nil,
  quote_empty:        true,
  force_quotes:       false,
  write_converters:   nil,
  write_nil_value:    nil,
  write_empty_value:  "",
  strip:              false,
}

Options for Parsing¶ ↑

Option col_sep¶ ↑

Specifies the String field separator to be used for both parsing and generating. The String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:col_sep) # => "," (comma)

For examples in this section:

ary = ['a', 'b', 'c']

Using the default:

str = CSV.generate_line(line)
str # => "a,b,c\n"
ary = CSV.parse_line(str)
ary # => ["a", "b", "c"]

Using : (colon):

col_sep = ':'
str = CSV.generate_line(ary, col_sep: col_sep)
str # => "a:b:c\n"
ary = CSV.parse_line(str, col_sep: col_sep)
ary # => [["a", "b", "c"]]

Using :: (two colons):

col_sep = '::'
str = CSV.generate_line(ary, col_sep: col_sep)
str # => "a::b::c\n"
ary = CSV.parse_line(str, col_sep: col_sep)
ary # => [["a", "b", "c"]]

Raises an exception if given the empty String:

col_sep = ''
# Raises ArgumentError (:col_sep must be 1 or more characters: "")
CSV.parse_line("a:b:c\n", col_sep: col_sep)

Raises an exception if the given value is not String-convertible:

col_sep = BasicObject.new
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.generate_line(line, col_sep: col_sep)
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.parse(str, col_sep: col_sep)

Option row_sep¶ ↑

Specifies the row separator, a String or the Symbol :auto (see below), to be used for both parsing and generating.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:row_sep) # => :auto

When row_sep is a String, that String becomes the row separator. The String will be transcoded into the data’s Encoding before use.

Using "\n":

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using | (pipe):

row_sep = '|'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0|bar,1|baz,2|"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using -- (two hyphens):

row_sep = '--'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0--bar,1--baz,2--"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

row_sep = ''
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0bar,1baz,2"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0bar", "1baz", "2"]]

When row_sep is the Symbol :auto (the default), invokes auto-discovery of the row separator.

Auto-discovery reads ahead in the data looking for the next \r\n, \n, or \r sequence. The sequence will be selected even if it occurs in a quoted field, assuming that you would have the same line endings there.

row_sep = :auto
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

The default $INPUT_RECORD_SEPARATOR ($/) is used if any of the following is true:

• None of those sequences is found.

• Data is ARGF, STDIN, STDOUT, or STDERR.

• The stream is only available for output.

Obviously, discovery takes a little time. Set manually if speed is important. Also note that IO objects should be opened in binary mode on Windows if this feature will be used as the line-ending translation can cause problems with resetting the document position to where it was before the read ahead.

Raises an exception if the given value is not String-convertible:

row_sep = BasicObject.new
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.generate_line(ary, row_sep: row_sep)
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.parse(str, row_sep: row_sep)

Option quote_char¶ ↑

Specifies the character (String of length 1) used used to quote fields in both parsing and generating. This String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_char) # => "\"" (backslash)

This is useful for an application that incorrectly uses ' (single-quote) to quote fields, instead of the correct " (double-quote).

Using the default:

ary = ['a', 'b', '"c"', 'd']
str = CSV.generate_line(ary)
str # => "a,b,\"\"\"c\"\"\",d\n"
ary = CSV.parse_line(str)
ary # => ["a", "b", "\"c\"", "d"]

Using ' (single-quote):

quote_char = "'"
ary = ['a', 'b', '\'c\'', 'd']
str = CSV.generate_line(ary, quote_char: quote_char)
str # => "a,b,'''c''',d\n"
ary = CSV.parse_line(str, quote_char: quote_char)
ary # => [["a", "b", "'c'", "d"]]

Raises an exception if the String length is greater than 1:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: 'xx')

Option field_size_limit¶ ↑

Specifies the Integer field size limit.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:field_size_limit) # => nil

This is a maximum size CSV will read ahead looking for the closing quote for a field. (In truth, it reads to the first line ending beyond this size.) If a quote cannot be found within the limit CSV will raise a MalformedCSVError, assuming the data is faulty. You can use this limit to prevent what are effectively DoS attacks on the parser. However, this limit can cause a legitimate parse to fail; therefore the default value is nil (no limit).

For the examples in this section:

str = <<~EOT
  "a","b"
  "
  2345
  ",""
EOT
str # => "\"a\",\"b\"\n\"\n2345\n\",\"\"\n"

Using the default nil:

ary = CSV.parse(str)
ary # => [["a", "b"], ["\n2345\n", ""]]

Using 50:

field_size_limit = 50
ary = CSV.parse(str, field_size_limit: field_size_limit)
ary # => [["a", "b"], ["\n2345\n", ""]]

Raises an exception if a field is too long:

big_str = "123456789\n" * 1024
# Raises CSV::MalformedCSVError (Field size exceeded in line 1.)
CSV.parse('valid,fields,"' + big_str + '"', field_size_limit: 2048)

Option converters¶ ↑

Specifies a single field converter name or Proc, or an Array of field converter names and Procs.

See Field Converters

Default value:

CSV::DEFAULT_OPTIONS.fetch(:converters) # => nil

The value may be a single field converter name:

str = '1,2,3'
# Without a converter
ary = CSV.parse_line(str)
ary # => ["1", "2", "3"]
# With built-in converter :integer
ary = CSV.parse_line(str, converters: :integer)
ary # => [1, 2, 3]

The value may be an Array of field converter names:

str = '1,3.14159'
# Without converters
ary = CSV.parse_line(str)
ary # => ["1", "3.14159"]
# With built-in converters
ary = CSV.parse_line(str, converters: [:integer, :float])
ary # => [1, 3.14159]

The value may be a Proc custom converter:

str = ' foo  ,  bar  ,  baz  '
# Without a converter
ary = CSV.parse_line(str)
ary # => [" foo  ", "  bar  ", "  baz  "]
# With a custom converter
ary = CSV.parse_line(str, converters: proc {|field| field.strip })
ary # => ["foo", "bar", "baz"]

See also Custom Converters

Raises an exception if the converter is not a converter name or a Proc:

str = 'foo,0'
# Raises NoMethodError (undefined method `arity' for nil:NilClass)
CSV.parse(str, converters: :foo)

Option unconverted_fields¶ ↑

Specifies the boolean that determines whether unconverted field values are to be available.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:unconverted_fields) # => nil

The unconverted field values are those found in the source data, prior to any conversions performed via option converters.

When option unconverted_fields is true, each returned row (Array or CSV::Row) has an added method, unconverted_fields, that returns the unconverted field values:

str = <<-EOT
foo,0
bar,1
baz,2
EOT
# Without unconverted_fields
csv = CSV.parse(str, converters: :integer)
csv # => [["foo", 0], ["bar", 1], ["baz", 2]]
csv.first.respond_to?(:unconverted_fields) # => false
# With unconverted_fields
csv = CSV.parse(str, converters: :integer, unconverted_fields: true)
csv # => [["foo", 0], ["bar", 1], ["baz", 2]]
csv.first.respond_to?(:unconverted_fields) # => true
csv.first.unconverted_fields # => ["foo", "0"]

Option headers¶ ↑

Specifies a boolean, Symbol, Array, or String to be used to define column headers.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:headers) # => false

Without headers:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str)
csv # => #<CSV io_type:StringIO encoding:UTF-8 lineno:0 col_sep:"," row_sep:"\n" quote_char:"\"">
csv.headers # => nil
csv.shift # => ["Name", "Count"]

If set to true or the Symbol :first_row, the first row of the data is treated as a row of headers:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: true)
csv # => #<CSV io_type:StringIO encoding:UTF-8 lineno:2 col_sep:"," row_sep:"\n" quote_char:"\"" headers:["Name", "Count"]>
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">

If set to an Array, the Array elements are treated as headers:

str = <<-EOT
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: ['Name', 'Count'])
csv
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">

If set to a String str, method CSV::parse_line(str, options) is called with the current options, and the returned Array is treated as headers:

str = <<-EOT
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: 'Name,Count')
csv
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">

Option return_headers¶ ↑

Specifies the boolean that determines whether method shift returns or ignores the header row.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:return_headers) # => false

Examples:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
# Without return_headers first row is str.
csv = CSV.new(str, headers: true)
csv.shift # => #<CSV::Row "Name":"foo" "Count":"0">
# With return_headers first row is headers.
csv = CSV.new(str, headers: true, return_headers: true)
csv.shift # => #<CSV::Row "Name":"Name" "Count":"Count">

Option header_converters¶ ↑

Specifies a String converter name or an Array of converter names.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:header_converters) # => nil

Identical in functionality to option converters except that:

• The converters apply only to the header row.

• The built-in header converters are :downcase and :symbol.

Examples:

str = <<-EOT
foo,0
bar,1
baz,2
EOT
headers = ['Name', 'Value']
# With no header converter
csv = CSV.parse(str, headers: headers)
csv.headers # => ["Name", "Value"]
# With header converter :downcase
csv = CSV.parse(str, headers: headers, header_converters: :downcase)
csv.headers # => ["name", "value"]
# With header converter :symbol
csv = CSV.parse(str, headers: headers, header_converters: :symbol)
csv.headers # => [:name, :value]
# With both
csv = CSV.parse(str, headers: headers, header_converters: [:downcase, :symbol])
csv.headers # => [:name, :value]

Option skip_blanks¶ ↑

Specifies a boolean that determines whether blank lines in the input will be ignored; a line that contains a column separator is not considered to be blank.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:skip_blanks) # => false

See also option skiplines.

For examples in this section:

str = <<-EOT
foo,0

bar,1
baz,2

,
EOT

Using the default, false:

ary = CSV.parse(str)
ary # => [["foo", "0"], [], ["bar", "1"], ["baz", "2"], [], [nil, nil]]

Using true:

ary = CSV.parse(str, skip_blanks: true)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"], [nil, nil]]

Using a truthy value:

ary = CSV.parse(str, skip_blanks: :foo)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"], [nil, nil]]

Option skip_lines¶ ↑

Specifies an object to use in identifying comment lines in the input that are to be ignored:

• If a Regexp, ignores lines that match it.

• If a String, converts it to a Regexp, ignores lines that match it.

• If nil, no lines are considered to be comments.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:skip_lines) # => nil

For examples in this section:

str = <<-EOT
# Comment
foo,0
bar,1
baz,2
# Another comment
EOT
str # => "# Comment\nfoo,0\nbar,1\nbaz,2\n# Another comment\n"

Using the default, nil:

ary = CSV.parse(str)
ary # => [["# Comment"], ["foo", "0"], ["bar", "1"], ["baz", "2"], ["# Another comment"]]

Using a Regexp:

ary = CSV.parse(str, skip_lines: /^#/)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using a String:

ary = CSV.parse(str, skip_lines: '#')
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Raises an exception if given an object that is not a Regexp, a String, or nil:

# Raises ArgumentError (:skip_lines has to respond to #match: 0)
CSV.parse(str, skip_lines: 0)

Option liberal_parsing¶ ↑

Specifies the boolean value that determines whether CSV will attempt to parse input not conformant with RFC 4180, such as double quotes in unquoted fields.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:liberal_parsing) # => false

For examples in this section:

str = 'is,this "three, or four",fields'

Without liberal_parsing:

# Raises CSV::MalformedCSVError (Illegal quoting in str 1.)
CSV.parse_line(str)

With liberal_parsing:

ary = CSV.parse_line(str, liberal_parsing: true)
ary # => ["is", "this \"three", " or four\"", "fields"]

Option nil_value¶ ↑

Specifies the object that is to be substituted for each null (no-text) field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:nil_value) # => nil

With the default, nil:

CSV.parse_line('a,,b,,c') # => ["a", nil, "b", nil, "c"]

With a different object:

CSV.parse_line('a,,b,,c', nil_value: 0) # => ["a", 0, "b", 0, "c"]

Option empty_value¶ ↑

Specifies the object that is to be substituted for each field that has an empty String.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:empty_value) # => "" (empty string)

With the default, "":

CSV.parse_line('a,"",b,"",c') # => ["a", "", "b", "", "c"]

With a different object:

CSV.parse_line('a,"",b,"",c', empty_value: 'x') # => ["a", "x", "b", "x", "c"]

Options for Generating¶ ↑

Option col_sep¶ ↑

Specifies the String field separator to be used for both parsing and generating. The String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:col_sep) # => "," (comma)

For examples in this section:

ary = ['a', 'b', 'c']

Using the default:

str = CSV.generate_line(line)
str # => "a,b,c\n"
ary = CSV.parse_line(str)
ary # => ["a", "b", "c"]

Using : (colon):

col_sep = ':'
str = CSV.generate_line(ary, col_sep: col_sep)
str # => "a:b:c\n"
ary = CSV.parse_line(str, col_sep: col_sep)
ary # => [["a", "b", "c"]]

Using :: (two colons):

col_sep = '::'
str = CSV.generate_line(ary, col_sep: col_sep)
str # => "a::b::c\n"
ary = CSV.parse_line(str, col_sep: col_sep)
ary # => [["a", "b", "c"]]

Raises an exception if given the empty String:

col_sep = ''
# Raises ArgumentError (:col_sep must be 1 or more characters: "")
CSV.parse_line("a:b:c\n", col_sep: col_sep)

Raises an exception if the given value is not String-convertible:

col_sep = BasicObject.new
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.generate_line(line, col_sep: col_sep)
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.parse(str, col_sep: col_sep)

Option row_sep¶ ↑

Specifies the row separator, a String or the Symbol :auto (see below), to be used for both parsing and generating.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:row_sep) # => :auto

When row_sep is a String, that String becomes the row separator. The String will be transcoded into the data’s Encoding before use.

Using "\n":

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using | (pipe):

row_sep = '|'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0|bar,1|baz,2|"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using -- (two hyphens):

row_sep = '--'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0--bar,1--baz,2--"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

row_sep = ''
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0bar,1baz,2"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0bar", "1baz", "2"]]

When row_sep is the Symbol :auto (the default), invokes auto-discovery of the row separator.

Auto-discovery reads ahead in the data looking for the next \r\n, \n, or \r sequence. The sequence will be selected even if it occurs in a quoted field, assuming that you would have the same line endings there.

row_sep = :auto
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

The default $INPUT_RECORD_SEPARATOR ($/) is used if any of the following is true:

• None of those sequences is found.

• Data is ARGF, STDIN, STDOUT, or STDERR.

• The stream is only available for output.

Obviously, discovery takes a little time. Set manually if speed is important. Also note that IO objects should be opened in binary mode on Windows if this feature will be used as the line-ending translation can cause problems with resetting the document position to where it was before the read ahead.

Raises an exception if the given value is not String-convertible:

row_sep = BasicObject.new
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.generate_line(ary, row_sep: row_sep)
# Raises NoMethodError (undefined method `to_s' for #<BasicObject:>)
CSV.parse(str, row_sep: row_sep)

Option quote_char¶ ↑

Specifies the character (String of length 1) used used to quote fields in both parsing and generating. This String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_char) # => "\"" (backslash)

This is useful for an application that incorrectly uses ' (single-quote) to quote fields, instead of the correct " (double-quote).

Using the default:

ary = ['a', 'b', '"c"', 'd']
str = CSV.generate_line(ary)
str # => "a,b,\"\"\"c\"\"\",d\n"
ary = CSV.parse_line(str)
ary # => ["a", "b", "\"c\"", "d"]

Using ' (single-quote):

quote_char = "'"
ary = ['a', 'b', '\'c\'', 'd']
str = CSV.generate_line(ary, quote_char: quote_char)
str # => "a,b,'''c''',d\n"
ary = CSV.parse_line(str, quote_char: quote_char)
ary # => [["a", "b", "'c'", "d"]]

Raises an exception if the String length is greater than 1:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: 'xx')

Option write_headers¶ ↑

Specifies the boolean that determines whether a header row is included in the output; ignored if there are no headers.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_headers) # => nil

Without write_headers:

file_path = 't.csv'
CSV.open(file_path,'w',
    :headers => ['Name','Value']
  ) do |csv|
    csv << ['foo', '0']
end
CSV.open(file_path) do |csv|
  csv.shift
end # => ["foo", "0"]

With write_headers“:

CSV.open(file_path,'w',
    :write_headers=> true,
    :headers => ['Name','Value']
  ) do |csv|
    csv << ['foo', '0']
end
CSV.open(file_path) do |csv|
  csv.shift
end # => ["Name", "Value"]

Option force_quotes¶ ↑

Specifies the boolean that determines whether each output field is to be double-quoted.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:force_quotes) # => false

For examples in this section:

ary = ['foo', 0, nil]

Using the default, false:

str = CSV.generate_line(ary)
str # => "foo,0,\n"

Using true:

str = CSV.generate_line(ary, force_quotes: true)
str # => "\"foo\",\"0\",\"\"\n"

Option quote_empty¶ ↑

Specifies the boolean that determines whether an empty value is to be double-quoted.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_empty) # => true

With the default true:

CSV.generate_line(['"', ""]) # => "\"\"\"\",\"\"\n"

With false:

CSV.generate_line(['"', ""], quote_empty: false) # => "\"\"\"\",\n"

Option write_converters¶ ↑

Specifies the Proc or Array of Procs that are to be called for converting each output field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_converters) # => nil

With no write converter:

str = CSV.generate_line(["\na\n", "\tb\t", " c "])
str # => "\"\na\n\",\tb\t, c \n"

With a write converter:

strip_converter = lambda {|field| field.strip }
str = CSV.generate_line(["\na\n", "\tb\t", " c "], write_converters: strip_converter)
str # => "a,b,c\n"

With two write converters (called in order):

upcase_converter = lambda {|field| field.upcase }
downcase_converter = lambda {|field| field.downcase }
write_converters = [upcase_converter, downcase_converter]
str = CSV.generate_line(['a', 'b', 'c'], write_converters: write_converters)
str # => "a,b,c\n"

Raises an exception if the converter returns a value that is neither nil nor String-convertible:

bad_converter = lambda {|field| BasicObject.new }
# Raises NoMethodError (undefined method `is_a?' for #<BasicObject:>)
CSV.generate_line(['a', 'b', 'c'], write_converters: bad_converter)#

Option write_nil_value¶ ↑

Specifies the object that is to be substituted for each nil field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_nil_value) # => nil

Without the option:

str = CSV.generate_line(['a', nil, 'c', nil])
str # => "a,,c,\n"

With the option:

str = CSV.generate_line(['a', nil, 'c', nil], write_nil_value: "x")
str # => "a,x,c,x\n"

Option write_empty_value¶ ↑

Specifies the object that is to be substituted for each field that has an empty String.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_empty_value) # => ""

Without the option:

str = CSV.generate_line(['a', '', 'c', ''])
str # => "a,\"\",c,\"\"\n"

With the option:

str = CSV.generate_line(['a', '', 'c', ''], write_empty_value: "x")
str # => "a,x,c,x\n"

Option strip¶ ↑

Specifies the boolean value that determines whether whitespace is stripped from each input field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:strip) # => false

With default value false:

ary = CSV.parse_line(' a , b ')
ary # => [" a ", " b "]

With value true:

ary = CSV.parse_line(' a , b ', strip: true)
ary # => ["a", "b"]

CSV with headers¶ ↑

CSV allows to specify column names of CSV file, whether they are in data, or provided separately. If headers are specified, reading methods return an instance of CSV::Table, consisting of CSV::Row.

# Headers are part of data
data = CSV.parse(<<~ROWS, headers: true)
  Name,Department,Salary
  Bob,Engineering,1000
  Jane,Sales,2000
  John,Management,5000
ROWS

data.class      #=> CSV::Table
data.first      #=> #<CSV::Row "Name":"Bob" "Department":"Engineering" "Salary":"1000">
data.first.to_h #=> {"Name"=>"Bob", "Department"=>"Engineering", "Salary"=>"1000"}

# Headers provided by developer
data = CSV.parse('Bob,Engineering,1000', headers: %i[name department salary])
data.first      #=> #<CSV::Row name:"Bob" department:"Engineering" salary:"1000">

CSV Converters¶ ↑

By default, each field parsed by CSV is formed into a String. You can use a converter to convert certain fields into other Ruby objects.

When you specify a converter for parsing, each parsed field is passed to the converter; its return value becomes the new value for the field. A converter might, for example, convert an integer embedded in a String into a true Integer. (In fact, that’s what built-in field converter :integer does.)

There are additional built-in converters, and custom converters are also supported.

All converters try to transcode fields to UTF-8 before converting. The conversion will fail if the data cannot be transcoded, leaving the field unchanged.

Field Converters¶ ↑

There are three ways to use field converters; these examples use built-in field converter :integer, which converts each parsed integer string to a true Integer.

Option converters with a singleton parsing method:

ary = CSV.parse_line('0,1,2', converters: :integer)
ary # => [0, 1, 2]

Option converters with a new CSV instance:

csv = CSV.new('0,1,2', converters: :integer)
# Field converters in effect:
csv.converters # => [:integer]
csv.shift # => [0, 1, 2]

Method convert adds a field converter to a CSV instance:

csv = CSV.new('0,1,2')
# Add a converter.
csv.convert(:integer)
csv.converters # => [:integer]
csv.shift # => [0, 1, 2]

The built-in field converters are in Hash CSV::Converters. The Symbol keys there are the names of the converters:

CSV::Converters.keys # => [:integer, :float, :numeric, :date, :date_time, :all]

Converter :integer converts each field that +Integer()+ accepts:

data = '0,1,2,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["0", "1", "2", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :integer)
csv # => [0, 1, 2, "x"]

Converter :float converts each field that +Float()+ accepts:

data = '1.0,3.14159,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["1.0", "3.14159", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :float)
csv # => [1.0, 3.14159, "x"]

Converter :numeric converts with both :integer and :float..

Converter :date converts each field that +Date::parse()+ accepts:

data = '2001-02-03,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["2001-02-03", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :date)
csv # => [#<Date: 2001-02-03 ((2451944j,0s,0n),+0s,2299161j)>, "x"]

Converter :date_time converts each field that +DateTime::parse() accepts:

data = '2020-05-07T14:59:00-05:00,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["2020-05-07T14:59:00-05:00", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :date_time)
csv # => [#<DateTime: 2020-05-07T14:59:00-05:00 ((2458977j,71940s,0n),-18000s,2299161j)>, "x"]

Converter :numeric converts with both :date_time and :numeric..

As seen above, method convert adds converters to a CSV instance, and method converters returns an Array of the converters in effect:

csv = CSV.new('0,1,2')
csv.converters # => []
csv.convert(:integer)
csv.converters # => [:integer]
csv.convert(:date)
csv.converters # => [:integer, :date]

You can add a custom field converter to Hash CSV::Converters:

strip_converter = proc {|field| field.strip}
CSV::Converters[:strip] = strip_converter
CSV::Converters.keys # => [:integer, :float, :numeric, :date, :date_time, :all, :strip]

Then use it to convert fields:

str = ' foo , 0 '
ary = CSV.parse_line(str, converters: :strip)
ary # => ["foo", "0"]

See Custom Converters.

Header Converters¶ ↑

Header converters operate only on headers (and not on other rows).

There are three ways to use header converters; these examples use built-in header converter :dowhcase, which downcases each parsed header.

Option header_converters with a singleton parsing method:

str = "Name,Count\nFoo,0\n,Bar,1\nBaz,2"
tbl = CSV.parse(str, headers: true, header_converters: :downcase)
tbl.class # => CSV::Table
tbl.headers # => ["name", "count"]

Option header_converters with a new CSV instance:

csv = CSV.new(str, header_converters: :downcase)
# Header converters in effect:
csv.header_converters # => [:downcase]
tbl = CSV.parse(str, headers: true)
tbl.headers # => ["Name", "Count"]

Method header_convert adds a header converter to a CSV instance:

csv = CSV.new(str)
# Add a header converter.
csv.header_convert(:downcase)
csv.header_converters # => [:downcase]
tbl = CSV.parse(str, headers: true)
tbl.headers # => ["Name", "Count"]

The built-in header converters are in Hash CSV::Converters. The Symbol keys there are the names of the converters:

CSV::HeaderConverters.keys # => [:downcase, :symbol]

Converter :downcase converts each header by downcasing it:

str = "Name,Count\nFoo,0\n,Bar,1\nBaz,2"
tbl = CSV.parse(str, headers: true, header_converters: :downcase)
tbl.class # => CSV::Table
tbl.headers # => ["name", "count"]

Converter :symbol by making it into a Symbol:

str = "Name,Count\nFoo,0\n,Bar,1\nBaz,2"
tbl = CSV.parse(str, headers: true, header_converters: :symbol)
tbl.headers # => [:name, :count]

Details:

• Strips leading and trailing whitespace.

• Downcases the header.

• Replaces embedded spaces with underscores.

• Removes non-word characters.

• Makes the string into a Symbol.

You can add a custom header converter to Hash CSV::HeaderConverters:

strip_converter = proc {|field| field.strip}
CSV::HeaderConverters[:strip] = strip_converter
CSV::HeaderConverters.keys # => [:downcase, :symbol, :strip]

Then use it to convert headers:

str = " Name , Value \nfoo,0\nbar,1\nbaz,2"
tbl = CSV.parse(str, headers: true, header_converters: :strip)
tbl.headers # => ["Name", "Value"]

See Custom Converters.

Custom Converters¶ ↑

You can define custom converters.

The converter is a Proc that is called with two arguments, String field and CSV::FieldInfo field_info; it returns a String that will become the field value:

converter = proc {|field, field_info| <some_string> }

To illustrate:

converter = proc {|field, field_info| p [field, field_info]; field}
ary = CSV.parse_line('foo,0', converters: converter)

Produces:

["foo", #<struct CSV::FieldInfo index=0, line=1, header=nil>]
["0", #<struct CSV::FieldInfo index=1, line=1, header=nil>]

In each of the output lines:

• The first Array element is the passed String field.

• The second is a FieldInfo structure containing information about the field:

  • The 0-based column index.

  • The 1-based line number.

  • The header for the column, if available.

If the converter does not need field_info, it can be omitted:

converter = proc {|field| ... }

CSV and Character Encodings (M17n or Multilingualization)¶ ↑

This new CSV parser is m17n savvy. The parser works in the Encoding of the IO or String object being read from or written to. Your data is never transcoded (unless you ask Ruby to transcode it for you) and will literally be parsed in the Encoding it is in. Thus CSV will return Arrays or Rows of Strings in the Encoding of your data. This is accomplished by transcoding the parser itself into your Encoding.

Some transcoding must take place, of course, to accomplish this multiencoding support. For example, :col_sep, :row_sep, and :quote_char must be transcoded to match your data. Hopefully this makes the entire process feel transparent, since CSV’s defaults should just magically work for your data. However, you can set these values manually in the target Encoding to avoid the translation.

It’s also important to note that while all of CSV’s core parser is now Encoding agnostic, some features are not. For example, the built-in converters will try to transcode data to UTF-8 before making conversions. Again, you can provide custom converters that are aware of your Encodings to avoid this translation. It’s just too hard for me to support native conversions in all of Ruby’s Encodings.

Anyway, the practical side of this is simple: make sure IO and String objects passed into CSV have the proper Encoding set and everything should just work. CSV methods that allow you to open IO objects (CSV::foreach(), CSV::open(), CSV::read(), and CSV::readlines()) do allow you to specify the Encoding.

One minor exception comes when generating CSV into a String with an Encoding that is not ASCII compatible. There’s no existing data for CSV to use to prepare itself and thus you will probably need to manually specify the desired Encoding for most of those cases. It will try to guess using the fields in a row of output though, when using CSV::generate_line() or Array#to_csv().

I try to point out any other Encoding issues in the documentation of methods as they come up.

This has been tested to the best of my ability with all non-“dummy” Encodings Ruby ships with. However, it is brave new code and may have some bugs. Please feel free to report any issues you find with it.