diff options
Diffstat (limited to 'python3.html.markdown')
| -rw-r--r-- | python3.html.markdown | 129 |
1 files changed, 95 insertions, 34 deletions
diff --git a/python3.html.markdown b/python3.html.markdown index 37987582..d09c2819 100644 --- a/python3.html.markdown +++ b/python3.html.markdown @@ -6,6 +6,9 @@ contributors: - ["Andre Polykanine", "https://github.com/Oire"] - ["Zachary Ferguson", "http://github.com/zfergus2"] - ["evuez", "http://github.com/evuez"] + - ["Rommel Martinez", "https://ebzzry.io"] + - ["Roberto Fernandez Diaz", "https://github.com/robertofd1995"] + - ["caminsha", "https://github.com/caminsha"] filename: learnpython3.py --- @@ -39,10 +42,10 @@ Note: This article applies to Python 3 specifically. Check out [here](http://lea 10 * 2 # => 20 35 / 5 # => 7.0 -# Result of integer division truncated down both for positive and negative. +# Integer division rounds down for both positive and negative numbers. 5 // 3 # => 1 -5.0 // 3.0 # => 1.0 # works on floats too -5 // 3 # => -2 +5.0 // 3.0 # => 1.0 # works on floats too -5.0 // 3.0 # => -2.0 # The result of division is always a float @@ -70,15 +73,24 @@ not False # => True True and False # => False False or True # => True -# Note using Bool operators with ints -# False is 0 and True is 1 +# True and False are actually 1 and 0 but with different keywords +True + True # => 2 +True * 8 # => 8 +False - 5 # => -5 + +# Comparison operators look at the numerical value of True and False +0 == False # => True +1 == True # => True +2 == True # => False +-5 != False # => True + +# Using boolean logical operators on ints casts them to booleans for evaluation, but their non-cast value is returned # Don't mix up with bool(ints) and bitwise and/or (&,|) +bool(0) # => False +bool(4) # => True +bool(-6) # => True 0 and 2 # => 0 -5 or 0 # => -5 -0 == False # => True -2 == True # => False -1 == True # => True --5 != False != True #=> True # Equality is == 1 == 1 # => True @@ -94,7 +106,10 @@ False or True # => True 2 <= 2 # => True 2 >= 2 # => True -# Comparisons can be chained! +# Seeing whether a value is in a range +1 < 2 and 2 < 3 # => True +2 < 3 and 3 < 2 # => False +# Chaining makes this look nicer 1 < 2 < 3 # => True 2 < 3 < 2 # => False @@ -137,6 +152,12 @@ len("This is a string") # => 16 # still use the old style of formatting: "%s can be %s the %s way" % ("Strings", "interpolated", "old") # => "Strings can be interpolated the old way" +# You can also format using f-strings or formatted string literals (in Python 3.6+) +name = "Reiko" +f"She said her name is {name}." # => "She said her name is Reiko" +# You can basically put any Python statement inside the braces and it will be output in the string. +f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long." + # None is an object None # => None @@ -208,15 +229,11 @@ li[4] # Raises an IndexError # You can look at ranges with slice syntax. # The start index is included, the end index is not # (It's a closed/open range for you mathy types.) -li[1:3] # => [2, 4] -# Omit the end -li[2:] # => [4, 3] -# Omit the beginning -li[:3] # => [1, 2, 4] -# Select every second entry -li[::2] # =>[1, 4] -# Return a reversed copy of the list -li[::-1] # => [3, 4, 2, 1] +li[1:3] # Return list from index 1 to 3 => [2, 4] +li[2:] # Return list starting from index 2 => [4, 3] +li[:3] # Return list from beginning until index 3 => [1, 2, 4] +li[::2] # Return list selecting every second entry => [1, 4] +li[::-1] # Return list in reverse order => [3, 4, 2, 1] # Use any combination of these to make advanced slices # li[start:end:step] @@ -273,7 +290,8 @@ a, b, c = (1, 2, 3) # a is now 1, b is now 2 and c is now 3 # You can also do extended unpacking a, *b, c = (1, 2, 3, 4) # a is now 1, b is now [2, 3] and c is now 4 # Tuples are created by default if you leave out the parentheses -d, e, f = 4, 5, 6 +d, e, f = 4, 5, 6 # tuple 4, 5, 6 is unpacked into variables d, e and f +# respectively such that d = 4, e = 5 and f = 6 # Now look how easy it is to swap two values e, d = d, e # d is now 5 and e is now 4 @@ -293,16 +311,19 @@ valid_dict = {(1,2,3):[1,2,3]} # Values can be of any type, however. filled_dict["one"] # => 1 # Get all keys as an iterable with "keys()". We need to wrap the call in list() -# to turn it into a list. We'll talk about those later. Note - Dictionary key -# ordering is not guaranteed. Your results might not match this exactly. -list(filled_dict.keys()) # => ["three", "two", "one"] +# to turn it into a list. We'll talk about those later. Note - for Python +# versions <3.7, dictionary key ordering is not guaranteed. Your results might +# not match the example below exactly. However, as of Python 3.7, dictionary +# items maintain the order at which they are inserted into the dictionary. +list(filled_dict.keys()) # => ["three", "two", "one"] in Python <3.7 +list(filled_dict.keys()) # => ["one", "two", "three"] in Python 3.7+ # Get all values as an iterable with "values()". Once again we need to wrap it # in list() to get it out of the iterable. Note - Same as above regarding key # ordering. -list(filled_dict.values()) # => [3, 2, 1] - +list(filled_dict.values()) # => [3, 2, 1] in Python <3.7 +list(filled_dict.values()) # => [1, 2, 3] in Python 3.7+ # Check for existence of keys in a dictionary with "in" "one" in filled_dict # => True @@ -347,6 +368,8 @@ valid_set = {(1,), 1} # Add one more item to the set filled_set = some_set filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5} +# Sets do not have duplicate elements +filled_set.add(5) # it remains as before {1, 2, 3, 4, 5} # Do set intersection with & other_set = {3, 4, 5, 6} @@ -436,8 +459,19 @@ prints: """ for i in range(4, 8, 2): print(i) + +""" +To loop over a list, and retrieve both the index and the value of each item in the list +prints: + 0 dog + 1 cat + 2 mouse """ +list = ["dog", "cat", "mouse"] +for i, value in enumerate(list): + print(i, value) +""" While loops go until a condition is no longer met. prints: 0 @@ -468,6 +502,26 @@ with open("myfile.txt") as f: for line in f: print(line) +# Writing to a file +contents = {"aa": 12, "bb": 21} +with open("myfile1.txt", "w+") as file: + file.write(str(contents)) # writes a string to a file + +with open("myfile2.txt", "w+") as file: + file.write(json.dumps(contents)) # writes an object to a file + +# Reading from a file +with open('myfile1.txt', "r+") as file: + contents = file.read() # reads a string from a file +print(contents) +# print: {"aa": 12, "bb": 21} + +with open('myfile2.txt', "r+") as file: + contents = json.load(file) # reads a json object from a file +print(contents) +# print: {"aa": 12, "bb": 21} + + # Python offers a fundamental abstraction called the Iterable. # An iterable is an object that can be treated as a sequence. # The object returned by the range function, is an iterable. @@ -497,8 +551,14 @@ next(our_iterator) # => "three" # After the iterator has returned all of its data, it raises a StopIteration exception next(our_iterator) # Raises StopIteration -# You can grab all the elements of an iterator by calling list() on it. -list(filled_dict.keys()) # => Returns ["one", "two", "three"] +# We can also loop over it, in fact, "for" does this implicitly! +our_iterator = iter(our_iterable) +for i in our_iterator: + print(i) # Prints one, two, three + +# You can grab all the elements of an iterable or iterator by calling list() on it. +list(our_iterable) # => Returns ["one", "two", "three"] +list(our_iterator) # => Returns [] because state is saved #################################################### @@ -546,9 +606,9 @@ all_the_args(1, 2, a=3, b=4) prints: # Use * to expand tuples and use ** to expand kwargs. args = (1, 2, 3, 4) kwargs = {"a": 3, "b": 4} -all_the_args(*args) # equivalent to foo(1, 2, 3, 4) -all_the_args(**kwargs) # equivalent to foo(a=3, b=4) -all_the_args(*args, **kwargs) # equivalent to foo(1, 2, 3, 4, a=3, b=4) +all_the_args(*args) # equivalent to all_the_args(1, 2, 3, 4) +all_the_args(**kwargs) # equivalent to all_the_args(a=3, b=4) +all_the_args(*args, **kwargs) # equivalent to all_the_args(1, 2, 3, 4, a=3, b=4) # Returning multiple values (with tuple assignments) def swap(x, y): @@ -782,6 +842,7 @@ class Superhero(Human): # add additional class attributes: self.fictional = True self.movie = movie + # be aware of mutable default values, since defaults are shared self.superpowers = superpowers # The "super" function lets you access the parent class's methods @@ -789,11 +850,11 @@ class Superhero(Human): # This calls the parent class constructor: super().__init__(name) - # overload the sing method + # override the sing method def sing(self): return 'Dun, dun, DUN!' - # add an additional class method + # add an additional instance method def boast(self): for power in self.superpowers: print("I wield the power of {pow}!".format(pow=power)) @@ -816,7 +877,7 @@ if __name__ == '__main__': # Calls parent method but uses its own class attribute print(sup.get_species()) # => Superhuman - # Calls overloaded method + # Calls overridden method print(sup.sing()) # => Dun, dun, DUN! # Calls method from Human @@ -871,7 +932,7 @@ class Batman(Superhero, Bat): def __init__(self, *args, **kwargs): # Typically to inherit attributes you have to call super: - #super(Batman, self).__init__(*args, **kwargs) + # super(Batman, self).__init__(*args, **kwargs) # However we are dealing with multiple inheritance here, and super() # only works with the next base class in the MRO list. # So instead we explicitly call __init__ for all ancestors. @@ -900,7 +961,7 @@ if __name__ == '__main__': # Calls parent method but uses its own class attribute print(sup.get_species()) # => Superhuman - # Calls overloaded method + # Calls overridden method print(sup.sing()) # => nan nan nan nan nan batman! # Calls method from Human, because inheritance order matters |