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-rw-r--r--python3.html.markdown71
1 files changed, 65 insertions, 6 deletions
diff --git a/python3.html.markdown b/python3.html.markdown
index b3acb122..5a992b54 100644
--- a/python3.html.markdown
+++ b/python3.html.markdown
@@ -4,6 +4,7 @@ contributors:
- ["Louie Dinh", "http://pythonpracticeprojects.com"]
- ["Steven Basart", "http://github.com/xksteven"]
- ["Andre Polykanine", "https://github.com/Oire"]
+ - ["Zachary Ferguson", "http://github.com/zfergus2"]
filename: learnpython3.py
---
@@ -36,7 +37,7 @@ Note: This article applies to Python 3 specifically. Check out [here](http://lea
8 - 1 # => 7
10 * 2 # => 20
-# Except division which returns floats by default
+# Except division which returns floats, real numbers, by default
35 / 5 # => 7.0
# Result of integer division truncated down both for positive and negative.
@@ -51,13 +52,13 @@ Note: This article applies to Python 3 specifically. Check out [here](http://lea
# Modulo operation
7 % 3 # => 1
-# Exponentiation (x to the yth power)
+# Exponentiation (x**y, x to the yth power)
2**4 # => 16
# Enforce precedence with parentheses
(1 + 3) * 2 # => 8
-# Boolean values are primitives
+# Boolean values are primitives (Note: the capitalization)
True
False
@@ -95,6 +96,16 @@ False or True #=> True
1 < 2 < 3 # => True
2 < 3 < 2 # => False
+# (is vs. ==) is checks if two variable refer to the same object, but == checks
+# if the objects pointed to have the same values.
+a = [1, 2, 3, 4] # Point a at a new list, [1, 2, 3, 4]
+b = a # Point b at what a is pointing to
+b is a # => True, a and b refer to the same object
+b == a # => True, a's and b's objects are equal
+b = [1, 2, 3, 4] # Point a at a new list, [1, 2, 3, 4]
+b is a # => False, a and b do not refer to the same object
+b == a # => True, a's and b's objects are equal
+
# Strings are created with " or '
"This is a string."
'This is also a string.'
@@ -145,6 +156,10 @@ bool({}) #=> False
# Python has a print function
print("I'm Python. Nice to meet you!")
+# By default the print function also prints out a newline at the end.
+# Use the optional argument end to change the end character.
+print("Hello, World", end="!") # => Hello, World!
+
# No need to declare variables before assigning to them.
# Convention is to use lower_case_with_underscores
some_var = 5
@@ -191,6 +206,9 @@ li[::-1] # => [3, 4, 2, 1]
# Use any combination of these to make advanced slices
# li[start:end:step]
+# Make a one layer deep copy using slices
+li2 = li[:] # => li2 = [1, 2, 4, 3] but (li2 is li) will result in false.
+
# Remove arbitrary elements from a list with "del"
del li[2] # li is now [1, 2, 3]
@@ -213,6 +231,12 @@ tup = (1, 2, 3)
tup[0] # => 1
tup[0] = 3 # Raises a TypeError
+# Note that a tuple of length one has to have a comma after the last element but
+# tuples of other lengths, even zero, do not.
+type((1)) # => <class 'int'>
+type((1,)) # => <class 'tuple'>
+type(()) # => <class 'tuple'>
+
# You can do most of the list operations on tuples too
len(tup) # => 3
tup + (4, 5, 6) # => (1, 2, 3, 4, 5, 6)
@@ -232,6 +256,12 @@ empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}
+# Note keys for dictionaries have to be immutable types. This is to ensure that
+# the key can be converted to a constant hash value for quick look-ups.
+# Immutable types include ints, floats, strings, tuples.
+invalid_dict = {[1,2,3]: "123"} # => Raises a TypeError: unhashable type: 'list'
+valid_dict = {(1,2,3):[1,2,3]} # Values can be of any type, however.
+
# Look up values with []
filled_dict["one"] # => 1
@@ -278,6 +308,10 @@ empty_set = set()
# Initialize a set with a bunch of values. Yeah, it looks a bit like a dict. Sorry.
some_set = {1, 1, 2, 2, 3, 4} # some_set is now {1, 2, 3, 4}
+# Similar to keys of a dictionary, elements of a set have to be immutable.
+invalid_set = {[1], 1} # => Raises a TypeError: unhashable type: 'list'
+valid_set = {(1,), 1}
+
# Can set new variables to a set
filled_set = some_set
@@ -299,6 +333,7 @@ filled_set | other_set # => {1, 2, 3, 4, 5, 6}
10 in filled_set # => False
+
####################################################
## 3. Control Flow and Iterables
####################################################
@@ -352,6 +387,18 @@ for i in range(4, 8):
print(i)
"""
+"range(lower, upper, step)" returns an iterable of numbers
+from the lower number to the upper number, while incrementing
+by step. If step is not indicated, the default value is 1.
+prints:
+ 4
+ 6
+ 8
+"""
+for i in range(4, 8, 2):
+ print(i)
+"""
+
While loops go until a condition is no longer met.
prints:
0
@@ -376,7 +423,7 @@ else: # Optional clause to the try/except block. Must follow all except blocks
print("All good!") # Runs only if the code in try raises no exceptions
finally: # Execute under all circumstances
print("We can clean up resources here")
-
+
# Instead of try/finally to cleanup resources you can use a with statement
with open("myfile.txt") as f:
for line in f:
@@ -464,6 +511,15 @@ 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)
+# Returning multiple values (with tuple assignments)
+def swap(x, y):
+ return y, x # Return multiple values as a tuple without the parenthesis.
+ # (Note: parenthesis have been excluded but can be included)
+
+x = 1
+y = 2
+x, y = swap(x, y) # => x = 2, y = 1
+# (x, y) = swap(x,y) # Again parenthesis have been excluded but can be included.
# Function Scope
x = 5
@@ -494,10 +550,13 @@ add_10(3) # => 13
# There are also anonymous functions
(lambda x: x > 2)(3) # => True
+(lambda x, y: x ** 2 + y ** 2)(2, 1) # => 5
# TODO - Fix for iterables
# There are built-in higher order functions
map(add_10, [1, 2, 3]) # => [11, 12, 13]
+map(max, [1, 2, 3], [4, 2, 1]) # => [4, 2, 3]
+
filter(lambda x: x > 5, [3, 4, 5, 6, 7]) # => [6, 7]
# We can use list comprehensions for nice maps and filters
@@ -510,8 +569,8 @@ filter(lambda x: x > 5, [3, 4, 5, 6, 7]) # => [6, 7]
####################################################
-# We subclass from object to get a class.
-class Human(object):
+# We use the "class" operator to get a class
+class Human:
# A class attribute. It is shared by all instances of this class
species = "H. sapiens"