It's very useful for beginners to know why join is a string method.
It's very strange at the beginning, but very useful after this.
The result of join is always a string, but the object to be joined can be of many types [generators, list, tuples, etc].
.join is faster because it allocates memory only once. Better
than classical concatenation [see, extended explanation].
Once you learn it, it's very comfortable and you can do tricks like this to add parentheses.
>>> ",".join["12345"].join[["[","]"]]
Out:
'[1,2,3,4,5]'
>>> list = ["[","]"]
>>> ",".join["12345"].join[list]
Out:
'[1,2,3,4,5]'
This article describes how to concatenate strings in Python.
- Concatenate multiple strings:
+,+=operator - Concatenate strings and numbers:
+,+=operator,str[],format[], f-string - Concatenate a list of strings into one string:
join[] - Concatenate a list of numbers into one string:
join[],str[]
Concatenate multiple strings: +, +=
operator
+ operator
You can concatenate string literals ['...' or "..."] and string variables with the + operator.
s = 'aaa' + 'bbb' + 'ccc'
print[s]
# aaabbbccc
s1 = 'aaa'
s2 = 'bbb'
s3 = 'ccc'
s = s1 + s2 + s3
print[s]
# aaabbbccc
s = s1 + s2 + s3 + 'ddd'
print[s]
# aaabbbcccddd
+= operator
You can append another string to a string with the in-place operator, +=. The string on the right is concatenated after the string variable on the left.
s1 += s2
print[s1]
# aaabbb
If you want to add a string to the end of a string variable, use the +=
operator.
s = 'aaa'
s += 'xxx'
print[s]
# aaaxxx
Concatenate by writing string literals consecutively
If you write string literals consecutively, they are concatenated.
s = 'aaa''bbb''ccc'
print[s]
# aaabbbccc
Even if there are multiple spaces or newlines with backslash \ [considered as continuation lines] between the strings, they are concatenated.
s = 'aaa' 'bbb' 'ccc'
print[s]
# aaabbbccc
s = 'aaa'\
'bbb'\
'ccc'
print[s]
# aaabbbccc
Using this, you can write long strings on multiple lines in the code.
- Write a long string on multiple lines in Python
You cannot do this for string variables.
# s = s1 s2 s3
# SyntaxError: invalid syntax
Concatenate strings and numbers: +, += operator, str[], format[], f-string
The + operation between different types raises an error.
s1 = 'aaa'
s2 = 'bbb'
i = 100
f = 0.25
# s = s1 + i
# TypeError: must be str, not int
If you want to concatenate a string and a number,
such as an integer int or a floating point float, convert the number to a string with str[] and then use the + operator or += operator.
s = s1 + '_' + str[i] + '_' + s2 + '_' + str[f]
print[s]
# aaa_100_bbb_0.25
Use the format[] function or the string method format[] if you want to convert the number format, such as zero padding or decimal places.
- string - Format Specification Mini-Language — Python 3.8.1 documentation
s = s1 + '_' + format[i, '05'] + '_' + s2 + '_' + format[f, '.5f']
print[s]
# aaa_00100_bbb_0.25000
s = '{}_{:05}_{}_{:.5f}'.format[s1, i, s2, f]
print[s]
# aaa_00100_bbb_0.25000
Of course, it is also possible to embed the value of a variable directly in a string without specifying the format, which is simpler than using the + operator.
s = '{}_{}_{}_{}'.format[s1, i, s2, f]
print[s]
# aaa_100_bbb_0.25
In Python 3.6 and later, you can also use a formatted string literal [f-string]. It is even simpler than format[].
- 2. Lexical analysis - Formatted string literals — Python 3.9.4 documentation
s = f'{s1}_{i:05}_{s2}_{f:.5f}'
print[s]
# aaa_00100_bbb_0.25000
s = f'{s1}_{i}_{s2}_{f}'
print[s]
# aaa_100_bbb_0.25
Concatenate a list of strings into one string: join[]
You can concatenate a list of strings into a single string with the string method, join[].
- Built-in Types - str - join[] — Python 3.8.1 documentation
Call the join[] method from 'String to insert' and pass
[List of strings].
'String to insert'.join[[List of strings]]
If you use an empty string '', [List of strings] is simply concatenated, and if you use a comma ,, it makes a comma-delimited string. If a newline character \n is used, a newline will be inserted for each string.
l = ['aaa', 'bbb', 'ccc']
s = ''.join[l]
print[s]
# aaabbbccc
s = ','.join[l]
print[s]
# aaa,bbb,ccc
s = '-'.join[l]
print[s]
# aaa-bbb-ccc
s = '\n'.join[l]
print[s]
# aaa
# bbb
# ccc
Note that other iterable objects such as tuples can be specified as arguments of join[].
Use split[] to split a string separated by a specific delimiter and get it as a list. See the
following article for details.
- Split strings in Python [delimiter, line break, regex, etc.]
Concatenate a list of numbers into one string: join[], str[]
If you set a non-string list to join[], an error is raised.
l = [0, 1, 2]
# s = '-'.join[l]
# TypeError: sequence item 0: expected str instance, int found
If you want to concatenate a list of numbers [int or float] into a
single string, apply the str[] function to each element in the list comprehension to convert numbers to strings, then concatenate them with join[].
s = '-'.join[[str[n] for n in l]]
print[s]
# 0-1-2
It can be written as a generator expression, a generator version of list comprehensions. Generator expressions are enclosed in parentheses [], but you can omit [] if the generator expression is the only argument of a function or method.
s = '-'.join[[str[n] for n in l]]
print[s]
# 0-1-2
s = '-'.join[str[n] for n in l]
print[s]
# 0-1-2
In general, generator expressions have the
advantage of reduced memory usage compared with list comprehensions. However, since join[] internally converts a generator into a list, there is no advantage to using generator expressions.
- python - List vs generator comprehension speed with join function - Stack Overflow
See the following article for details on list comprehensions and generator expressions.
- List comprehensions in Python