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David Beazley
2020-05-26 11:40:09 -05:00
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Notes/01_Introduction/04_Strings.md
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# 1.4 Strings
This section covers the basics of text manipulation.
### Representing Text
String are text literals written in programs with quotes.
@@ -40,9 +38,8 @@ at the keyboard. Here are some common escape codes:
### String Representation
Each character represents a raw unicode code point.
<!-- TODO: Add Picture of following characters -->
The characters in a string are Unicode and represent a so-called "code-point". You can
specify an exact code-point using the following escape sequences:
```python
a = '\xf1' # a = 'ñ'
@@ -51,7 +48,12 @@ c = '\U0001D122' # c = '𝄢'
d = '\N{FORALL}' # d = '∀'
```
Strings work like an array for accessing its characters. You use an integer index, starting at 0.
The [Unicode Character Database](https://unicode.org/charts) is a reference for all
available character codes.
### String Indexing
Strings work like an array for accessing individual characters. You use an integer index, starting at 0.
```python
a = 'Hello world'
@@ -85,7 +87,7 @@ len(s) # 5
# Membership test (`in`, `not in`)
t = 'e' in s # True
f = 'x' in s # False
tt = 'hi' not in s # True
g = 'hi' not in s # True
# Replication (s * n)
rep = s * 5 # 'HelloHelloHelloHelloHello'
@@ -95,14 +97,14 @@ rep = s * 5 # 'HelloHelloHelloHelloHello'
Strings have methods that perform various operations with the string data.
Stripping any leading / trailing white space.
Example: stripping any leading / trailing white space.
```python
s = ' Hello '
t = s.strip() # 'Hello'
```
Case conversion.
Example: Case conversion.
```python
s = 'Hello'
@@ -110,7 +112,7 @@ l = s.lower() # 'hello'
u = s.upper() # 'HELLO'
```
Replacing text.
Example: Replacing text.
```python
s = 'Hello world'
@@ -119,7 +121,8 @@ t = s.replace('Hello' , 'Hallo') # 'Hallo world'
**More string methods:**
Strings have a wide variety of other methods for testing and manipulating the text data:
Strings have a wide variety of other methods for testing and manipulating the text data.
This is small sample of methods:
```python
s.endswith(suffix) # Check if string ends with suffix
@@ -158,7 +161,7 @@ TypeError: 'str' object does not support item assignment
### String Conversions
Use `str()` to convert a value to a string.
Use `str()` to convert any value to a string suitable for printing.
```python
>>> x = 42
@@ -169,13 +172,13 @@ Use `str()` to convert a value to a string.
### Byte Strings
A string of 8-bit bytes.
A string of 8-bit bytes, commonly encountered with low-level I/O.
```python
data = b'Hello World\r\n'
```
By putting a little b before our string literal we specify that it is a byte string as opposed to a text string.
By putting a little b before the first quotation, you specify that it is a byte string as opposed to a text string.
Most of the usual string operations work.
@@ -185,13 +188,13 @@ data[0:5] # b'Hello'
data.replace(b'Hello', b'Cruel') # b'Cruel World\r\n'
```
Indexing is a bit different because it returns byte values.
Indexing is a bit different because it returns byte values as integers.
```python
data[0] # 72 (ASCII code for 'H')
```
Conversion to/from text.
Conversion to/from text strings.
```python
text = data.decode('utf-8') # bytes -> text
@@ -200,7 +203,7 @@ data = text.encode('utf-8') # text -> bytes
### Raw Strings
Raw strings are strings with uninterpreted backslash. They are little by prefixing the initial quote with a lowercase "r".
Raw strings are string literals with an uninterpreted backslash. They specified by prefixing the initial quote with a lowercase "r".
```python
>>> rs = r'c:\newdata\test' # Raw (uninterpreted backslash)
@@ -208,8 +211,9 @@ Raw strings are strings with uninterpreted backslash. They are little by prefix
'c:\\newdata\\test'
```
The string is the literal text, exactly as typed.
This is useful in situations where the backslash has special significance. Example: filename, regular expressions, etc.
The string is the literal text enclosed inside, exactly as typed.
This is useful in situations where the backslash has special
significance. Example: filename, regular expressions, etc.
### f-Strings
@@ -228,13 +232,13 @@ A string with formatted expression substitution.
>>>
```
**Note: This requires Python 3.6 or newer.**
**Note: This requires Python 3.6 or newer.** The meaning of the format codes
is covered later.
## Exercises
In this exercise, we experiment with operations on Python's string type.
You may want to do most of this exercise at the Python interactive prompt where you can easily see the results.
In these exercises, you experiment with operations on Python's string type.
You should do this at the Python interactive prompt where you can easily see the results.
Important note:
> In exercises where you are supposed to interact with the interpreter,
@@ -251,7 +255,7 @@ Start by defining a string containing a series of stock ticker symbols like this
>>>
```
### (a): Extracting individual characters and substrings
### Exercise 1.13: Extracting individual characters and substrings
Strings are arrays of characters. Try extracting a few characters:
@@ -269,7 +273,7 @@ Strings are arrays of characters. Try extracting a few characters:
>>>
```
### (b) Strings as read-only objects
### Exercise 1.14: Strings as read-only objects
In Python, strings are read-only.
@@ -283,7 +287,7 @@ TypeError: 'str' object does not support item assignment
>>>
```
### (c) String concatenation
### Exercise 1.15: String concatenation
Although string data is read-only, you can always reassign a variable
to a newly created string.
@@ -298,7 +302,7 @@ the end of `symbols`:
>>>
```
Oops! That's not what we wanted. Fix it so that the `symbols` variable holds the value `'HPQ,AAPL,IBM,MSFT,YHOO,SCO,GOOG'`.
Oops! That's not what you wanted. Fix it so that the `symbols` variable holds the value `'HPQ,AAPL,IBM,MSFT,YHOO,SCO,GOOG'`.
In these examples, it might look like the original string is being
modified, in an apparent violation of strings being read only. Not
@@ -307,15 +311,16 @@ time. When the variable name `symbols` is reassigned, it points to the
newly created string. Afterwards, the old string is destroyed since
it's not being used anymore.
### (d) Membership testing (substring testing)
### Exercise 1.16: Membership testing (substring testing)
Experiment with the `in` operator to check for substrings. At the interactive prompt, try these operations:
Experiment with the `in` operator to check for substrings. At the
interactive prompt, try these operations:
```pycon
>>> 'IBM' in symbols
?
>>> 'AA' in symbols
?
True
>>> 'CAT' in symbols
?
>>>
@@ -323,7 +328,7 @@ Experiment with the `in` operator to check for substrings. At the interactive p
*Why did the check for "AA" return `True`?*
### (e) String Methods
### Exercise 1.17: String Methods
At the Python interactive prompt, try experimenting with some of the string methods.
@@ -359,7 +364,7 @@ Try some more operations:
>>>
```
### (f) f-strings
### Exercise 1.18: f-strings
Sometimes you want to create a string and embed the values of
variables into it.
@@ -375,13 +380,15 @@ To do that, use an f-string. For example:
>>>
```
Modify the `mortgage.py` program from Exercise 1.3 to create its output using f-strings.
Modify the `mortgage.py` program from [Exercise 1.10](03_Numbers) to create its output using f-strings.
Try to make it so that output is nicely aligned.
### Commentary
As you start to experiment with the interpreter, you often want to know more about the operations supported by different objects.
For example, how do you find out what operations are available on a string?
As you start to experiment with the interpreter, you often want to
know more about the operations supported by different objects. For
example, how do you find out what operations are available on a
string?
Depending on your Python environment, you might be able to see a list
of available methods via tab-completion. For example, try typing
@@ -399,7 +406,12 @@ builtin-in `dir()` function. For example:
```python
>>> s = 'hello'
>>> dir(s)
['__add__', '__class__', '__contains__', ..., 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']
['__add__', '__class__', '__contains__', ..., 'find', 'format',
'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace',
'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition',
'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit',
'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase',
'title', 'translate', 'upper', 'zfill']
>>>
```
@@ -417,9 +429,4 @@ upper(...)
>>>
```
IDEs and alternative interactive shells often give you more help here.
For example, a popular alternative to Python's normal interactive mode
is IPython (http://ipython.org). IPython provides some nice features
such as tab-completion of method names, more integrated help and more.
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