• csv.writer()

  • csv.DictWriter()

14.2.1. SetUp

14.2.2. Sequence

  • tuple -> string

  • open(file, mode='wt', encoding='utf-8')

  • csv.writer(file)

  • writer.writerow(row)

>>> DATA = ('Alice', 'Apricot', 30)
>>>
>>> with open('/tmp/myfile.csv', mode='wt', encoding='utf-8') as file:
...     writer = csv.writer(file)
...     writer.writerow(DATA)
18

Result:

>>> print(open('/tmp/myfile.csv').read())
Alice,Apricot,30

14.2.3. List of Sequences

  • list[tuple] -> string

  • open(file, mode='wt', encoding='utf-8')

  • csv.writer(file)

  • writer.writerows(rows)

>>> DATA = [
...     ('firstname', 'lastname', 'age'),
...     ('Alice', 'Apricot', 30),
...     ('Bob', 'Blackthorn', 31),
...     ('Carol', 'Corn', 32),
...     ('Dave', 'Durian', 33),
...     ('Eve', 'Elderberry', 34),
...     ('Mallory', 'Melon', 15),
... ]
>>>
>>> with open('/tmp/myfile.csv', mode='wt', encoding='utf-8') as file:
...     writer = csv.writer(file)
...     writer.writerows(DATA)

Result:

>>> print(open('/tmp/myfile.csv').read())
firstname,lastname,age
Alice,Apricot,30
Bob,Blackthorn,31
Carol,Corn,32
Dave,Durian,33
Eve,Elderberry,34
Mallory,Melon,15

14.2.4. Mapping

  • dict -> string

  • open(file, mode='wt', encoding='utf-8')

  • csv.DictWriter(file, fieldnames)

  • result.writeheader()

  • result.writerow(DATA)

>>> DATA = {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 30}
>>>
>>>
>>> fieldnames = DATA.keys()
>>>
>>> with open('/tmp/myfile.csv', mode='wt', encoding='utf-8') as file:
...     result = csv.DictWriter(file, fieldnames)
...     result.writeheader()
...     result.writerow(DATA)
24
18

Result:

>>> print(open('/tmp/myfile.csv').read())
firstname,lastname,age
Alice,Apricot,30

14.2.5. List of Mappings

  • list[dict] -> string

  • fieldnames = sorted(set(str(key) for row in DATA for key in row.keys()))

  • csv.DictWriter(file, fieldnames)

  • result.writeheader()

  • result.writerows(DATA)

>>> DATA = [
...     {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 30},
...     {'firstname': 'Bob', 'lastname': 'Blackthorn', 'age': 31},
...     {'firstname': 'Carol', 'lastname': 'Corn', 'age': 32},
...     {'firstname': 'Dave', 'lastname': 'Durian', 'age': 33},
...     {'firstname': 'Eve', 'lastname': 'Elderberry', 'age': 34},
...     {'firstname': 'Mallory', 'lastname': 'Melon', 'age': 15},
... ]
>>>
>>>
>>> fieldnames = sorted(set(str(key) for row in DATA for key in row.keys()))
>>>
>>> with open('/tmp/myfile.csv', mode='wt', encoding='utf-8') as file:
...     result = csv.DictWriter(file, fieldnames)
...     result.writeheader()
...     result.writerows(DATA)
24

Result:

>>> print(open('/tmp/myfile.csv').read())
age,firstname,lastname
30,Alice,Apricot
31,Bob,Blackthorn
32,Carol,Corn
33,Dave,Durian
34,Eve,Elderberry
15,Mallory,Melon

14.2.6. List of Objects

  • vars(obj) converts obj to dict

>>> class User:
...     def __init__(self, firstname, lastname, age):
...         self.firstname = firstname
...         self.lastname = lastname
...         self.age = age
...
...     def __repr__(self):
...         clsname = self.__class__.__name__
...         firstname = self.firstname
...         lastname = self.lastname
...         age = self.age
...         return f'{clsname}({firstname=}, {lastname=}, {age=})'
>>>
>>>
>>> DATA = [
...     User('Alice', 'Apricot', age=30),
...     User('Bob', 'Blackthorn', age=31),
...     User('Carol', 'Corn', age=32),
...     User('Dave', 'Durian', age=33),
...     User('Eve', 'Elderberry', age=34),
...     User('Mallory', 'Melon', age=15),
... ]
>>>
>>> data = [vars(obj) for obj in DATA]
>>> fieldnames = sorted(set(str(key) for row in data for key in row.keys()))
>>>
>>> with open('/tmp/myfile.csv', mode='wt', encoding='utf-8') as file:
...     result = csv.DictWriter(file, fieldnames)
...     result.writeheader()
...     result.writerows(data)
24

Result:

>>> print(open('/tmp/myfile.csv').read())
age,firstname,lastname
30,Alice,Apricot
31,Bob,Blackthorn
32,Carol,Corn
33,Dave,Durian
34,Eve,Elderberry
15,Mallory,Melon

14.2.7. Assignments

# %% About
# - Name: CSV Writer Sequence
# - Difficulty: easy
# - Lines: 3
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Using `csv.writer()` save `DATA` to file
# 2. Use Unix `\n` line terminator
# 3. Run doctests - all must succeed

# %% Polish
# 1. Za pomocą `csv.writer()` zapisz `DATA` do pliku
# 2. Użyj zakończenia linii Unix `\n`
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# Alice,Apricot,30
# <BLANKLINE>

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> result = open(FILE).read()
>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is str, \
'Variable `result` has an invalid type; expected: `str`.'

>>> from os import remove
>>> remove(FILE)

>>> print(result)
Alice,Apricot,30
<BLANKLINE>
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import csv

# %% Types

# %% Data
FILE = r'_temporary.csv'

DATA = ('Alice', 'Apricot', 30)

# %% Result
with open(FILE, mode='wt', encoding='utf-8') as file:
    ...


# %% About
# - Name: CSV Writer ListSequence
# - Difficulty: easy
# - Lines: 3
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Using `csv.writer()` save `DATA` to file
# 2. Use Unix `\n` line terminator
# 3. Run doctests - all must succeed

# %% Polish
# 1. Za pomocą `csv.writer()` zapisz `DATA` do pliku
# 2. Użyj zakończenia linii Unix `\n`
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# firstname,lastname,age
# Alice,Apricot,30
# Bob,Blackthorn,31
# Carol,Corn,32
# Dave,Durian,33
# Eve,Elderberry,34
# Mallory,Melon,15
# <BLANKLINE>

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> result = open(FILE).read()
>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is str, \
'Variable `result` has an invalid type; expected: `str`.'

>>> from os import remove
>>> remove(FILE)

>>> print(result)
firstname,lastname,age
Alice,Apricot,30
Bob,Blackthorn,31
Carol,Corn,32
Dave,Durian,33
Eve,Elderberry,34
Mallory,Melon,15
<BLANKLINE>
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import csv

# %% Types

# %% Data
FILE = r'_temporary.csv'

DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Blackthorn', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
with open(FILE, mode='wt', encoding='utf-8') as file:
    ...


# %% About
# - Name: CSV DictWriter Mapping
# - Difficulty: easy
# - Lines: 5
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Write `DATA` to `FILE` in CSV format
# 2. Use `csv.DictWriter()`
# 3. Sort header (fieldnames)
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zapisz `DATA` do `FILE` w formacie CSV
# 2. Użyj `csv.DictWriter()`
# 3. Posortuj nagłówek (fieldnames)
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# firstname,lastname,age
# Alice,Apricot,30
# <BLANKLINE>

# %% Hints
# - `sorted()`
# - `dict.keys()`
# - `csv.DictWriter()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> result = open(FILE).read()
>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is str, \
'Variable `result` has an invalid type; expected: `str`.'

>>> from os import remove
>>> remove(FILE)

>>> print(result)
firstname,lastname,age
Alice,Apricot,30
<BLANKLINE>
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import csv

# %% Types

# %% Data
FILE = r'_temporary.csv'

DATA = {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 30}

# %% Result
with open(FILE, mode='wt', encoding='utf-8') as file:
    ...


# %% About
# - Name: CSV DictWriter ListMappings
# - Difficulty: easy
# - Lines: 5
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Write `DATA` to `FILE` in CSV format
# 2. Use `csv.DictWriter()`
# 3. Sort header (fieldnames)
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zapisz `DATA` do `FILE` w formacie CSV
# 2. Użyj `csv.DictWriter()`
# 3. Posortuj nagłówek (fieldnames)
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# age,firstname,lastname
# 30,Alice,Apricot
# 31,Bob,Blackthorn
# 32,Carol,Corn
# 33,Dave,Durian
# 34,Eve,Elderberry
# 15,Mallory,Melon
# <BLANKLINE>

# %% Hints
# - `sorted()`
# - `dict.keys()`
# - `csv.DictWriter()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> result = open(FILE).read()
>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is str, \
'Variable `result` has an invalid type; expected: `str`.'

>>> from os import remove
>>> remove(FILE)

>>> print(result)
age,firstname,lastname
30,Alice,Apricot
31,Bob,Blackthorn
32,Carol,Corn
33,Dave,Durian
34,Eve,Elderberry
15,Mallory,Melon
<BLANKLINE>
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import csv

# %% Types

# %% Data
FILE = r'_temporary.csv'

DATA = [
    {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 30},
    {'firstname': 'Bob', 'lastname': 'Blackthorn', 'age': 31},
    {'firstname': 'Carol', 'lastname': 'Corn', 'age': 32},
    {'firstname': 'Dave', 'lastname': 'Durian', 'age': 33},
    {'firstname': 'Eve', 'lastname': 'Elderberry', 'age': 34},
    {'firstname': 'Mallory', 'lastname': 'Melon', 'age': 15},
]

# %% Result
with open(FILE, mode='wt', encoding='utf-8') as file:
    ...


# %% About
# - Name: CSV DictWriter ListObjects
# - Difficulty: easy
# - Lines: 6
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Write `DATA` to `FILE` in CSV format
# 2. Use `csv.DictWriter()`
# 3. Sort header (fieldnames)
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zapisz `DATA` do `FILE` w formacie CSV
# 2. Użyj `csv.DictWriter()`
# 3. Posortuj nagłówek (fieldnames)
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# age,firstname,lastname
# 30,Alice,Apricot
# 31,Bob,Blackthorn
# 32,Carol,Corn
# 33,Dave,Durian
# 34,Eve,Elderberry
# 15,Mallory,Melon
# <BLANKLINE>

# %% Hints
# - `vars()`
# - `sorted()`
# - `dict.keys()`
# - `csv.DictWriter()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> result = open(FILE).read()
>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is str, \
'Variable `result` has an invalid type; expected: `str`.'

>>> from os import remove
>>> remove(FILE)

>>> print(result)
age,firstname,lastname
30,Alice,Apricot
31,Bob,Blackthorn
32,Carol,Corn
33,Dave,Durian
34,Eve,Elderberry
15,Mallory,Melon
<BLANKLINE>
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import csv

# %% Types

# %% Data
FILE = r'_temporary.csv'

class User:
    def __init__(self, firstname, lastname, age=None):
        self.firstname = firstname
        self.lastname = lastname
        self.age = age

    def __repr__(self):
        clsname = self.__class__.__qualname__
        arguments = ', '.join(f'{k}={v!r}' for k,v in vars(self).items())
        return f'{clsname}({arguments})'


DATA = [
    User('Alice', 'Apricot', age=30),
    User('Bob', 'Blackthorn', age=31),
    User('Carol', 'Corn', age=32),
    User('Dave', 'Durian', age=33),
    User('Eve', 'Elderberry', age=34),
    User('Mallory', 'Melon', age=15),
]

# %% Result
with open(FILE, mode='wt', encoding='utf-8') as file:
    ...