String methods in Python
String methods are one of the most common method types in Python. You often need to manipulate strings to extract information or fit a certain format. Python includes several string methods that are designed to do the most common and useful transformations.
String methods are part of the str type, meaning that the methods exist as string variables, or part of the string directly. For example, the method .title() returns the string in initial caps and can be used with a string directly:
print("temperatures and facts about the moon".title())
Output: Temperatures And Facts About The Moon
The same behavior and usage happens on a variable:
heading = "temperatures and facts about the moon"
heading_upper = heading.title()
print(heading_upper)
Split a string
A common string method is .split(). Without arguments, the method separates the string at every space. If you use the method this way, you create a list of every word or number that's separated by a space:
temperatures = "Daylight: 260 F Nighttime: -280 F"
temperatures_list = temperatures.split()
print(temperatures_list)
Output: ['Daylight:', '260', 'F', 'Nighttime:', '-280', 'F']
In this example, you're dealing with multiple lines, so the (implicit) newline character can be used to split the string at the end of each line, creating single lines:
temperatures = "Daylight: 260 F\n Nighttime: -280 F"
temperatures_list = temperatures.split('\n')
print(temperatures_list)
Output: ['Daylight: 260 F', 'Nighttime: -280 F']
This type of splitting becomes handy when you need a loop to process or extract information, or when you're loading data from a text file.
Search for a string
Some string methods can look for content before processing, without using a loop. Let's assume that you have two sentences that discuss temperatures on various planets and moons. However, you're interested only in temperatures that are related to our Moon. That is, if the sentences aren't talking about the Moon, they shouldn't be processed to extract information.
The simplest way to discover whether a given word, character, or group of characters exists in a string is to use the in operator:
print("Moon" in "This text will describe facts and challenges with space travel")
Output: False
print("Moon" in "This text will describe facts about the Moon")
Output: True
An approach to finding the position of a specific word in a string is to use the .find() method:
temperatures = """Saturn has a daytime temperature of -170 degrees Celsius, while Mars has -28 Celsius."""
print(temperatures.find("Moon"))
Output: -1
The .find() method returns a -1 when the word isn't found, or it returns the index (the number representing the place in the string). The following example shows how it would behave if you're searching for the word Mars:
temperatures = """Saturn has a daytime temperature of -170 degrees Celsius, while Mars has -28 Celsius."""
print(temperatures.find("Mars"))
Output: 64
64 is the position where "Mars" appears in the string.
Another way to search for content is to use the .count() method, which returns the total number of occurrences of a certain word in a string:
temperatures = """Saturn has a daytime temperature of -170 degrees Celsius, while Mars has -28 Celsius."""
print(temperatures.count("Mars"))
print(temperatures.count("Moon"))
1
0
Strings in Python are case-sensitive, which means that Moon and moon are considered different words. To do a case-insensitive comparison, you can convert a string to all lowercase letters by using the .lower() method:
print("The Moon And The Earth".lower())
Output: the moon and the earth
Similar to the .lower() method, strings have an .upper() method that does the opposite, converting every character to uppercase:
print("The Moon And The Earth".upper())
Output: THE MOON AND THE EARTH
Tip
When you're searching for and checking content, a more robust approach is to lowercase a string so that casing doesn't prevent a match. For example, if you're counting the number of times the word the appears, the method wouldn't count the times where The appears, even though they're both the same word. You can use the .lower() method to change all characters to lowercase.
Check content
There are times when you process text to extract information that's irregular in its presentation. For example, the following string is simpler to process than an unstructured paragraph:
temperatures = "Mars Average Temperature: -60 C"
To extract the average temperature on Mars, you can do well with the following methods:
temperatures = "Mars Average Temperature: -60 C"
parts = temperatures.split(':')
print(parts)
print(parts[-1])
['Mars average temperature', ' -60 C']
' -60 C'
The preceding code trusts that everything after the colon (:) is a temperature. The string is split at :, which produces a list of two items. Using [-1] on the list returns the last item, which is the temperature in this example.
If the text is irregular, you can't use the same splitting methods to get the value. You must loop over the items and check to see whether the values are of a certain type. Python has methods that help check the type of string:
mars_temperature = "The highest temperature on Mars is about 30 C"
for item in mars_temperature.split():
if item.isnumeric():
print(item)
Output: 30
Like the .isnumeric() method, .isdecimal() can check for strings that look like decimals.
Important
It might be surprising to learn that "-70".isnumeric() would return False. This is because all characters in the string would need to be numeric, and the dash (-) isn't numeric. If you need to check negative numbers in a string, the .isnumeric() method wouldn't work.
There are extra validations that you can apply on strings to check for values. For negative numbers, the dash is prefixed to the number, and that can be detected with the .startswith() method:
print("-60".startswith('-'))
Output: True
Similarly, the .endswith() method helps with verifying the last character of a string:
if "30 C".endswith("C"):
print("This temperature is in Celsius")
Output: This temperature is in Celsius
Transform text
There are other methods that help in situations where text needs to be transformed into something else. So far, you saw strings that can use C for Celsius and F for Fahrenheit. You can use the .replace() method to find and replace occurrences of a character or group of characters:
print("Saturn has a daytime temperature of -170 degrees Celsius, while Mars has -28 Celsius.".replace("Celsius", "C"))
Output: Saturn has a daytime temperature of -170 degrees C, while Mars has -28 C.
As mentioned earlier, .lower() is a good way to normalize text to do a case-insensitive search. Let's quickly check to see whether some text discusses temperatures:
text = "Temperatures on the Moon can vary wildly."
print("temperatures" in text)
Output: False
text = "Temperatures on the Moon can vary wildly."
print("temperatures" in text.lower())
Output: True
You might not need to do case-insensitive verification all the time, but lowercasing every letter is a good approach when the text uses mixed casing.
After you split the text and perform the transformations, you might need to join the parts together again. Just as the .split() method can separate characters, the .join() method can put them back together.
The .join() method requires an iterable (such as a Python list) as an argument, so its usage looks different from other string methods:
moon_facts = ["The Moon is drifting away from the Earth.", "On average, the Moon is moving about 4cm every year."]
print(' '.join(moon_facts))
Output: The Moon is drifting away from the Earth. On average, the Moon is moving about 4cm every year.
In this example, ' ' is used to join every item in the list.