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Add new example: Tricky strings
Closes https://github.com/satwikkansal/wtfpython/issues/54
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README.md
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README.md
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@ -423,14 +423,52 @@ SyntaxError: EOL while scanning string literal
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---
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### String interning
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### Strings can be tricky sometimes
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1\.
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```py
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>>> a = "some_string"
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>>> id(a)
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140420665652016
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>>> id("some" + "_" + "string") # Notice that both the ids are same.
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140420665652016
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```
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2\.
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```py
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>>> a = "wtf"
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>>> b = "wtf"
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>>> a is b
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True
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>>> a = "wtf!"
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>>> b = "wtf!"
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>>> a is b
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False
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```
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3\.
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```py
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>>> 'a' * 20 is 'aaaaaaaaaaaaaaaaaaaa'
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True
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>>> 'a' * 21 is 'aaaaaaaaaaaaaaaaaaaaa'
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```
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Makes sense, right?
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#### 💡 Explanation:
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+ Such behavior is due to CPython optimization (called string interning) that tries to use existing immutable objects in some cases rather than creating a new object every time.
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+ After being interned, many variables may point to the same string object in memory (thereby saving memory).
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+ In the snippets above, strings are implicity interned. The decison of when to implicitly intern a string is implementation dependent. There are some facts that can be used to guess if a string will be interned or not:
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* All length 0 and length 1 strings are interned.
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* Strings are interned at compile time (`'wtf'` will be interned but `''.join(['w', 't', 'f']` will not be interned)
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* Strings that are not composed of ascii letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`.
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---
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### `+=` is faster
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```py
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# using "+", three strings:
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>>> timeit.timeit("s1 = s1 + s2 + s3", setup="s1 = ' ' * 100000; s2 = ' ' * 100000; s3 = ' ' * 100000", number=100)
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0.25748300552368164
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@ -441,7 +479,6 @@ SyntaxError: EOL while scanning string literal
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#### 💡 Explanation:
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+ `+=` is faster than `+` for concatenating more than two strings because the first string (example, `s1` for `s1 += s2 + s3`) is not destroyed while calculating the complete string.
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+ Both the strings refer to the same object because of CPython optimization that tries to use existing immutable objects in some cases (implementation specific) rather than creating a new object every time. You can read more about this [here](https://stackoverflow.com/questions/24245324/about-the-changing-id-of-an-immutable-string).
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---
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