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Python - Dictionary Method

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• Python - Dictionary Method
  • Construcor
    • dict(keyword arguments)
    • dict.fromkeys(keys, value)
  • Manipulate item
    • setdefault(keyname, value)
    • update(iterable)
    • pop(keyname, defaultvalue)
    • popitem()
    • clear()
  • Access Items
    • get(keyname, value)
    • items()
    • keys()
    • values()
  • Copy Dict
    • copy()

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Construcor

dict(keyword arguments)

• creates a dictionary.

• Parameter

  • keyword arguments: Optional. As many keyword arguments you like, separated by comma: key = value, key = value …

print("\n--------- dict(keyword arguments) --------\n")

empty_dict = dict()
print(empty_dict)   # {}

x_dict = dict(brand="BMW", model="X5", year="1990")
print(x_dict)   # {'brand': 'BMW', 'model': 'X5', 'year': '1990'}

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dict.fromkeys(keys, value)

• returns a dictionary with the specified keys and the specified value.

• Parameter

  • keys: Required. An iterable specifying the keys of the new dictionary
  • value: Optional. The value for all keys. Default value is None

print("\n--------- dict.fromkeys(keys, value) --------\n")

key_list = ("brand", "model", "year")

x_dict = dict.fromkeys(key_list)    # using None as default value
print(x_dict)   # {'brand': None, 'year': None}

x_dict = dict.fromkeys(key_list, "")
print(x_dict)   # {'brand': '', 'year': ''}

default_value = "unknown"
x_dict = dict.fromkeys(key_list, default_value)
print(x_dict)   # {'brand': 'unknown', 'model': 'unknown', 'year': 'unknown'}

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Manipulate item

setdefault(keyname, value)

• returns the value of the item with the specified key.

• Parameter

  • keyname: Required. The keyname of the item you want to return the value from
  • value: Optional. If the key exist, this parameter has no effect. If the key does not exist, this value becomes the key’s value. Default value None.

print("\n--------- dictionary.setdefault(keyname, value) --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

print(car.setdefault("model", "Bronco"))
# Mustang, the default value is not appled when the key exists.

print(car)  # {'brand': 'Ford', 'model': 'Mustang', 'year': 1964}


print(car.setdefault("color", "white"))  # white
print(car)
# {'brand': 'Ford', 'model': 'Mustang', 'year': 1964, 'color': 'white'}

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update(iterable)

• inserts the specified items to the dictionary.

  • if the key does not exist, insert the new key-value pair.
  • Otherwise, update the value of the existing key.

• Parameter

  • iterable: A dictionary or an iterable object with key value pairs, that will be inserted to the dictionary

print("\n--------- dictionary.update(iterable) --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

# must be the dictionary type

# car.update("year") # ValueError: dictionary update sequence element #0 has length 1; 2 is required
# car.update("year", 1990) # TypeError: update expected at most 1 argument, got 2
# car.update(("year", 1990)) # ValueError: dictionary update sequence element #0 has length 4; 2 is required
# car.update({"year", 1990}) # TypeError: cannot convert dictionary update sequence element #0 to a sequence

car.update({"year": 1990})  # default value will not apply when the key exists.
print(car)  # {'brand': 'Ford', 'model': 'Mustang', 'year': 1990}

car.update({"color": "white"})
print(car) # {'brand': 'Ford', 'model': 'Mustang', 'year': 1990, 'color': 'white'}

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pop(keyname, defaultvalue)

• removes the specified item from the dictionary.

• Parameter

  • keyname: Required. The keyname of the item you want to remove
  • defaultvalue: Optional. A value to return if the specified key do not exist. If this parameter is not specified, and the no item with the specified key is found, an error is raised

• Return

  • The value of the removed item

print("\n--------- dictionary.pop(keyname, defaultvalue) --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

print(car.pop("brand"))  # Ford
print(car.pop("model", "Model A"))
# Mustang, default is no applied when the key exists
print(car)  # {'year': 1964}

print(car.pop("price", 15000))  # 15000
print(car)  # {'year': 1964}

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popitem()

• removes the item that was last inserted into the dictionary.

• Return

  • The removed item

print("\n--------- dictionary.popitem() --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

print(car.popitem())    # ('year', 1964)

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clear()

• removes all the elements from a dictionary.

print("\n--------- dictionary.clear() --------\n")

x_dict = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

x_dict.clear()
print(x_dict)   # {}

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Access Items

get(keyname, value)

• returns the value of the item with the specified key.

• Parameter

  • keyname: Required. The keyname of the item you want to return the value from
  • value Optional. A value to return if the specified key does not exist. Default value None

print("\n--------- dictionary.get(keyname, value) --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

print(car.get("model"))     # Mustang
print(car.get("model", "Model A"))
# Mustang, default value is not applied when the key exists.
print(car.get("price", 15000))  # 15000

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items()

• returns a dict_items object. The view object contains the key-value pairs of the dictionary, as tuples in a list.

print("\n--------- dictionary.items() --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

xItems = car.items()

print(type(xItems))  # <class 'dict_items'>
# dict_items([('brand', 'Ford'), ('model', 'Mustang'), ('year', 1964)])
print(xItems)
[print(itm) for itm in xItems]
# ('brand', 'Ford')
# ('model', 'Mustang')
# ('year', 1964)

[print(itm[0], itm[1]) for itm in xItems]
# brand Ford
# model Mustang
# year 1964

# pionter
car.update({'color': 'white'})
# dict_items([('brand', 'Ford'), ('model', 'Mustang'), ('year', 1964), ('color', 'white')])
print(xItems)

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keys()

• returns a dict_keys object. The view object contains the keys of the dictionary, as a list.

print("\n--------- dictionary.keys() --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

xItems = car.keys()

print(type(xItems))  # <class 'dict_keys'>
print(xItems)  # dict_keys(['brand', 'model', 'year'])

[print(itm, ':', car[itm]) for itm in xItems]
# brand : Ford
# model : Mustang
# year : 1964

# pointer
car.update({"color": "white"})
print(xItems)   # dict_keys(['brand', 'model', 'year', 'color'])

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values()

• returns a dict_values object that contains the values of the dictionary, as a list.
• It is a pointer: when the dict is updated, the existing dict_values object will update as well.

print("\n--------- dictionary.values() --------\n")

car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

val_itm = car.values()

print(type(val_itm))    # <class 'dict_values'>
print(val_itm)    # dict_values(['Ford', 'Mustang', 1964])

# a pointer
car.update({"color":"white"})

print(val_itm)    # dict_values(['Ford', 'Mustang', 1964, 'white'])

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Copy Dict

copy()

• returns a copy of the specified dictionary.

print("\n--------- dictionary.copy() --------\n")

x_dict = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

# deep copy
y_dict = x_dict.copy()

y_dict["brand"] = "BMW"
y_dict["model"] = "X5"

print(x_dict)   # {'brand': 'Ford', 'model': 'Mustang', 'year': 1964}
print(y_dict)   # {'brand': 'BMW', 'model': 'X5', 'year': 1964}

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