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Boston Python Workshop 3/Data types: Difference between revisions
→Strings are a lot like lists
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==Strings==
* Strings are bits of text, and contain characters like numbers, letters, whitespace, and punctuation.
* String are surrounded by quotes.
* Use triple-quotes (""") to create whitespace-preserving multi-line strings.
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>>> "Hello"
'Hello'
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====String concatenation====
<pre>
>>> "Hello" + "World"
HelloWorld
>>> "Hello" + "World" + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects
>>> "Hello" + "World" + str(1)
'HelloWorld1'
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====Printing strings====
<pre>
>>> print "Hello" + "World"
HelloWorld
>>> print "Hello", "World"
Hello World
>>> print "Hello", "World", 1
Hello World 1
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was US $1.4 million.
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====Types====
<pre>
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<type 'str'>
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==Booleans==
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* There are two booleans, <code>True</code> and <code>False</code>.
* Use booleans to make decisions.
====Containment with 'in' and 'not in'====
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else:
print "Too extreme for me."
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====Types====
<pre>
>>> type(True)
<type 'bool'>
>>> type(False)
<type 'bool'>
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* Use lists to store data where order matters.
* Lists are indexed starting with 0.
====List initialization====
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>>> my_string
'Hello Jessica'
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* One big way in which strings are different from lists is that lists are mutable (you can change them), and strings are immutable (you can't change them). To "change" a string you have to make a copy:
<pre>
>>> h = "Hello"
>>> h[0] = "J"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> h = "J" + h[1:]
>>> h
'Jello'
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>>> type(my_list)
<type 'list'>
</pre>
==Dictionaries==
* Use dictionaries to store key/value pairs.
* Dictionaries do not guarantee ordering.
* A given key can only have one value, but multiple keys can have the same value.
====Initialization====
<pre>
>>> my_dict = {}
>>> my_dict
{}
>>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"}
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}
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====Adding elements to a dictionary====
<pre>
>>> your_dict["Dora"] = "vanilla"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}
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====Accessing elements of a dictionary====
<pre>
>>> your_dict["Alice"]
'chocolate'
>>> your_dict.get("Alice")
'chocolate'
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<pre>
>>> your_dict["Eve"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'Eve'
>>> "Eve" in her_dict
False
>>> "Alice" in her_dict
True
>>> your_dict.get("Eve")
>>> person = your_dict.get("Eve")
>>> print person
None
>>> print type(person)
<type 'NoneType'>
>>> your_dict.get("Alice")
'coconut'
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====Changing elements of a dictionary====
<pre>
>>> your_dict["Alice"] = "coconut"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}
</pre>
====Types====
<pre>
>>> type(my_dict)
<type 'dict'>
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