Related
I would like to put an int into a string. This is what I am doing at the moment:
num = 40
plot.savefig('hanning40.pdf') #problem line
I have to run the program for several different numbers, so I'd like to do a loop. But inserting the variable like this doesn't work:
plot.savefig('hanning', num, '.pdf')
How do I insert a variable into a Python string?
See also
If you tried using + to concatenate a number with a string (or between strings, etc.) and got an error message, see How can I concatenate str and int objects?.
If you are trying to assemble a URL with variable data, do not use ordinary string formatting, because it is error-prone and more difficult than necessary. Specialized tools are available. See Add params to given URL in Python.
If you are trying to assemble a SQL query, do not use ordinary string formatting, because it is a major security risk. This is the cause of "SQL injection" which costs real companies huge amounts of money every year. See for example Python: best practice and securest way to connect to MySQL and execute queries for proper techniques.
If you just want to print (output) the string, you can prepare it this way first, or if you don't need the string for anything else, print each piece of the output individually using a single call to print. See How can I print multiple things (fixed text and/or variable values) on the same line, all at once? for details on both approaches.
Using f-strings:
plot.savefig(f'hanning{num}.pdf')
This was added in 3.6 and is the new preferred way.
Using str.format():
plot.savefig('hanning{0}.pdf'.format(num))
String concatenation:
plot.savefig('hanning' + str(num) + '.pdf')
Conversion Specifier:
plot.savefig('hanning%s.pdf' % num)
Using local variable names (neat trick):
plot.savefig('hanning%(num)s.pdf' % locals())
Using string.Template:
plot.savefig(string.Template('hanning${num}.pdf').substitute(locals()))
See also:
Fancier Output Formatting - The Python Tutorial
Python 3's f-Strings: An Improved String Formatting Syntax (Guide) - RealPython
With the introduction of formatted string literals ("f-strings" for short) in Python 3.6, it is now possible to write this with a briefer syntax:
>>> name = "Fred"
>>> f"He said his name is {name}."
'He said his name is Fred.'
With the example given in the question, it would look like this
plot.savefig(f'hanning{num}.pdf')
plot.savefig('hanning(%d).pdf' % num)
The % operator, when following a string, allows you to insert values into that string via format codes (the %d in this case). For more details, see the Python documentation:
printf-style String Formatting
You can use + as the normal string concatenation function as well as str().
"hello " + str(10) + " world" == "hello 10 world"
In general, you can create strings using:
stringExample = "someString " + str(someNumber)
print(stringExample)
plot.savefig(stringExample)
If you would want to put multiple values into the string you could make use of format
nums = [1,2,3]
plot.savefig('hanning{0}{1}{2}.pdf'.format(*nums))
Would result in the string hanning123.pdf. This can be done with any array.
Special cases
Depending on why variable data is being used with strings, the general-purpose approaches may not be appropriate.
If you need to prepare an SQL query
Do not use any of the usual techniques for assembling a string. Instead, use your SQL library's functionality for parameterized queries.
A query is code, so it should not be thought about like normal text. Using the library will make sure that any inserted text is properly escaped. If any part of the query could possibly come from outside the program in any way, that is an opportunity for a malevolent user to perform SQL injection. This is widely considered one of the important computer security problems, costing real companies huge amounts of money every year and causing problems for countless customers. Even if you think you know the data is "safe", there is no real upside to using any other approach.
The syntax will depend on the library you are using and is outside the scope of this answer.
If you need to prepare a URL query string
See Add params to given URL in Python. Do not do it yourself; there is no practical reason to make your life harder.
Writing to a file
While it's possible to prepare a string ahead of time, it may be simpler and more memory efficient to just write each piece of data with a separate .write call. Of course, non-strings will still need to be converted to string before writing, which may complicate the code. There is not a one-size-fits-all answer here, but choosing badly will generally not matter very much.
If you are simply calling print
The built-in print function accepts a variable number of arguments, and can take in any object and stringify it using str. Before trying string formatting, consider whether simply passing multiple arguments will do what you want. (You can also use the sep keyword argument to control spacing between the arguments.)
# display a filename, as an example
print('hanning', num, '.pdf', sep='')
Of course, there may be other reasons why it is useful for the program to assemble a string; so by all means do so where appropriate.
It's important to note that print is a special case. The only functions that work this way are ones that are explicitly written to work this way. For ordinary functions and methods, like input, or the savefig method of Matplotlib plots, we need to prepare a string ourselves.
Concatenation
Python supports using + between two strings, but not between strings and other types. To work around this, we need to convert other values to string explicitly: 'hanning' + str(num) + '.pdf'.
Template-based approaches
Most ways to solve the problem involve having some kind of "template" string that includes "placeholders" that show where information should be added, and then using some function or method to add the missing information.
f-strings
This is the recommended approach when possible. It looks like f'hanning{num}.pdf'. The names of variables to insert appear directly in the string. It is important to note that there is not actually such a thing as an "f-string"; it's not a separate type. Instead, Python will translate the code ahead of time:
>>> def example(num):
... return f'hanning{num}.pdf'
...
>>> import dis
>>> dis.dis(example)
2 0 LOAD_CONST 1 ('hanning')
2 LOAD_FAST 0 (num)
4 FORMAT_VALUE 0
6 LOAD_CONST 2 ('.pdf')
8 BUILD_STRING 3
10 RETURN_VALUE
Because it's a special syntax, it can access opcodes that aren't used in other approaches.
str.format
This is the recommended approach when f-strings aren't possible - mainly, because the template string needs to be prepared ahead of time and filled in later. It looks like 'hanning{}.pdf'.format(num), or 'hanning{num}.pdf'.format(num=num)'. Here, format is a method built in to strings, which can accept arguments either by position or keyword.
Particularly for str.format, it's useful to know that the built-in locals, globals and vars functions return dictionaries that map variable names to the contents of those variables. Thus, rather than something like '{a}{b}{c}'.format(a=a, b=b, c=c), we can use something like '{a}{b}{c}'.format(**locals()), unpacking the locals() dict.
str.format_map
This is a rare variation on .format. It looks like 'hanning{num}.pdf'.format_map({'num': num}). Rather than accepting keyword arguments, it accepts a single argument which is a mapping.
That probably doesn't sound very useful - after all, rather than 'hanning{num}.pdf'.format_map(my_dict), we could just as easily write 'hanning{num}.pdf'.format(**my_dict). However, this is useful for mappings that determine values on the fly, rather than ordinary dicts. In these cases, unpacking with ** might not work, because the set of keys might not be determined ahead of time; and trying to unpack keys based on the template is unwieldy (imagine: 'hanning{num}.pdf'.format(num=my_mapping[num]), with a separate argument for each placeholder).
string.Formatter
The string standard library module contains a rarely used Formatter class. Using it looks like string.Formatter().format('hanning{num}.pdf', num=num). The template string uses the same syntax again. This is obviously clunkier than just calling .format on the string; the motivation is to allow users to subclass Formatter to define a different syntax for the template string.
All of the above approaches use a common "formatting language" (although string.Formatter allows changing it); there are many other things that can be put inside the {}. Explaining how it works is beyond the scope of this answer; please consult the documentation. Do keep in mind that literal { and } characters need to be escaped by doubling them up. The syntax is presumably inspired by C#.
The % operator
This is a legacy way to solve the problem, inspired by C and C++. It has been discouraged for a long time, but is still supported. It looks like 'hanning%s.pdf' % num, for simple cases. As you'd expect, literal '%' symbols in the template need to be doubled up to escape them.
It has some issues:
It seems like the conversion specifier (the letter after the %) should match the type of whatever is being interpolated, but that's not actually the case. Instead, the value is converted to the specified type, and then to string from there. This isn't normally necessary; converting directly to string works most of the time, and converting to other types first doesn't help most of the rest of the time. So 's' is almost always used (unless you want the repr of the value, using 'r'). Despite that, the conversion specifier is a mandatory part of the syntax.
Tuples are handled specially: passing a tuple on the right-hand side is the way to provide multiple arguments. This is an ugly special case that's necessary because we aren't using function-call syntax. As a result, if you actually want to format a tuple into a single placeholder, it must be wrapped in a 1-tuple.
Other sequence types are not handled specially, and the different behaviour can be a gotcha.
string.Template
The string standard library module contains a rarely used Template class. Instances provide substitute and safe_substitute methods that work similarly to the built-in .format (safe_substitute will leave placeholders intact rather than raising an exception when the arguments don't match). This should also be considered a legacy approach to the problem.
It looks like string.Template('hanning$num.pdf').substitute(num=num), and is inspired by traditional Perl syntax. It's obviously clunkier than the .format approach, since a separate class has to be used before the method is available. Braces ({}) can be used optionally around the name of the variable, to avoid ambiguity. Similarly to the other methods, literal '$' in the template needs to be doubled up for escaping.
I had a need for an extended version of this: instead of embedding a single number in a string, I needed to generate a series of file names of the form 'file1.pdf', 'file2.pdf' etc. This is how it worked:
['file' + str(i) + '.pdf' for i in range(1,4)]
You can make dict and substitute variables in your string.
var = {"name": "Abdul Jalil", "age": 22}
temp_string = "My name is %(name)s. I am %(age)s years old." % var
Is it possible to do the opposite of eval() in python which treats inputs as literal strings? For example f(Node(2)) -> 'Node(2)' or if there's some kind of higher-order operator that stops interpreting the input like lisp's quote() operator.
The answer to does Python have quote is no, Python does not have quote. There's no homoiconicity, and no native infrastructure to represent the languages own source code.
Now for what is the opposite of eval: you seem to be missing some part the picture here. Python's eval is not the opposite of quote. There's three types values can be represented as: as unparsed strings, as expressions, and as values (of any type).
quote and LISP-style "eval" convert between expressions and values.
Parsing and pretty printing form another pair between expressions and strings.
Python's actual eval goes directly from strings to values, and repr goes back (in the sense of printing a parsable value1).
Since SO does not support drawing diagrams:
So if you are looking for the opposite of eval, it's either trivially repr, but that might not really help you, or it would be the composition of quote and pretty priniting, if there were quoting in Python.
This composition with string functions a bit cumbersome, which is why LISPs let the reader deal with parsing once and for all, and from that point on only go between expressions and values. I don't know any language that has such a "quote + print" function, actually (C#'s nameof comes close, but is restricted to names).
Some caveats about the diagram:
It does not commute: you have deal with pure values or disregard side effects, for one, and most importantly quote is not really a function in some sense, of course. It's a special form. That's why repr(x) is not the same as prettyprint(quote(x)), in general.
It's not depicting pairs of isomorphisms. Multiple expressions can eval to the same value etc.
1That's not always the case in reality, but that's what it's suppsedly there for.
I would like to put an int into a string. This is what I am doing at the moment:
num = 40
plot.savefig('hanning40.pdf') #problem line
I have to run the program for several different numbers, so I'd like to do a loop. But inserting the variable like this doesn't work:
plot.savefig('hanning', num, '.pdf')
How do I insert a variable into a Python string?
See also
If you tried using + to concatenate a number with a string (or between strings, etc.) and got an error message, see How can I concatenate str and int objects?.
If you are trying to assemble a URL with variable data, do not use ordinary string formatting, because it is error-prone and more difficult than necessary. Specialized tools are available. See Add params to given URL in Python.
If you are trying to assemble a SQL query, do not use ordinary string formatting, because it is a major security risk. This is the cause of "SQL injection" which costs real companies huge amounts of money every year. See for example Python: best practice and securest way to connect to MySQL and execute queries for proper techniques.
If you just want to print (output) the string, you can prepare it this way first, or if you don't need the string for anything else, print each piece of the output individually using a single call to print. See How can I print multiple things (fixed text and/or variable values) on the same line, all at once? for details on both approaches.
Using f-strings:
plot.savefig(f'hanning{num}.pdf')
This was added in 3.6 and is the new preferred way.
Using str.format():
plot.savefig('hanning{0}.pdf'.format(num))
String concatenation:
plot.savefig('hanning' + str(num) + '.pdf')
Conversion Specifier:
plot.savefig('hanning%s.pdf' % num)
Using local variable names (neat trick):
plot.savefig('hanning%(num)s.pdf' % locals())
Using string.Template:
plot.savefig(string.Template('hanning${num}.pdf').substitute(locals()))
See also:
Fancier Output Formatting - The Python Tutorial
Python 3's f-Strings: An Improved String Formatting Syntax (Guide) - RealPython
With the introduction of formatted string literals ("f-strings" for short) in Python 3.6, it is now possible to write this with a briefer syntax:
>>> name = "Fred"
>>> f"He said his name is {name}."
'He said his name is Fred.'
With the example given in the question, it would look like this
plot.savefig(f'hanning{num}.pdf')
plot.savefig('hanning(%d).pdf' % num)
The % operator, when following a string, allows you to insert values into that string via format codes (the %d in this case). For more details, see the Python documentation:
printf-style String Formatting
You can use + as the normal string concatenation function as well as str().
"hello " + str(10) + " world" == "hello 10 world"
In general, you can create strings using:
stringExample = "someString " + str(someNumber)
print(stringExample)
plot.savefig(stringExample)
If you would want to put multiple values into the string you could make use of format
nums = [1,2,3]
plot.savefig('hanning{0}{1}{2}.pdf'.format(*nums))
Would result in the string hanning123.pdf. This can be done with any array.
Special cases
Depending on why variable data is being used with strings, the general-purpose approaches may not be appropriate.
If you need to prepare an SQL query
Do not use any of the usual techniques for assembling a string. Instead, use your SQL library's functionality for parameterized queries.
A query is code, so it should not be thought about like normal text. Using the library will make sure that any inserted text is properly escaped. If any part of the query could possibly come from outside the program in any way, that is an opportunity for a malevolent user to perform SQL injection. This is widely considered one of the important computer security problems, costing real companies huge amounts of money every year and causing problems for countless customers. Even if you think you know the data is "safe", there is no real upside to using any other approach.
The syntax will depend on the library you are using and is outside the scope of this answer.
If you need to prepare a URL query string
See Add params to given URL in Python. Do not do it yourself; there is no practical reason to make your life harder.
Writing to a file
While it's possible to prepare a string ahead of time, it may be simpler and more memory efficient to just write each piece of data with a separate .write call. Of course, non-strings will still need to be converted to string before writing, which may complicate the code. There is not a one-size-fits-all answer here, but choosing badly will generally not matter very much.
If you are simply calling print
The built-in print function accepts a variable number of arguments, and can take in any object and stringify it using str. Before trying string formatting, consider whether simply passing multiple arguments will do what you want. (You can also use the sep keyword argument to control spacing between the arguments.)
# display a filename, as an example
print('hanning', num, '.pdf', sep='')
Of course, there may be other reasons why it is useful for the program to assemble a string; so by all means do so where appropriate.
It's important to note that print is a special case. The only functions that work this way are ones that are explicitly written to work this way. For ordinary functions and methods, like input, or the savefig method of Matplotlib plots, we need to prepare a string ourselves.
Concatenation
Python supports using + between two strings, but not between strings and other types. To work around this, we need to convert other values to string explicitly: 'hanning' + str(num) + '.pdf'.
Template-based approaches
Most ways to solve the problem involve having some kind of "template" string that includes "placeholders" that show where information should be added, and then using some function or method to add the missing information.
f-strings
This is the recommended approach when possible. It looks like f'hanning{num}.pdf'. The names of variables to insert appear directly in the string. It is important to note that there is not actually such a thing as an "f-string"; it's not a separate type. Instead, Python will translate the code ahead of time:
>>> def example(num):
... return f'hanning{num}.pdf'
...
>>> import dis
>>> dis.dis(example)
2 0 LOAD_CONST 1 ('hanning')
2 LOAD_FAST 0 (num)
4 FORMAT_VALUE 0
6 LOAD_CONST 2 ('.pdf')
8 BUILD_STRING 3
10 RETURN_VALUE
Because it's a special syntax, it can access opcodes that aren't used in other approaches.
str.format
This is the recommended approach when f-strings aren't possible - mainly, because the template string needs to be prepared ahead of time and filled in later. It looks like 'hanning{}.pdf'.format(num), or 'hanning{num}.pdf'.format(num=num)'. Here, format is a method built in to strings, which can accept arguments either by position or keyword.
Particularly for str.format, it's useful to know that the built-in locals, globals and vars functions return dictionaries that map variable names to the contents of those variables. Thus, rather than something like '{a}{b}{c}'.format(a=a, b=b, c=c), we can use something like '{a}{b}{c}'.format(**locals()), unpacking the locals() dict.
str.format_map
This is a rare variation on .format. It looks like 'hanning{num}.pdf'.format_map({'num': num}). Rather than accepting keyword arguments, it accepts a single argument which is a mapping.
That probably doesn't sound very useful - after all, rather than 'hanning{num}.pdf'.format_map(my_dict), we could just as easily write 'hanning{num}.pdf'.format(**my_dict). However, this is useful for mappings that determine values on the fly, rather than ordinary dicts. In these cases, unpacking with ** might not work, because the set of keys might not be determined ahead of time; and trying to unpack keys based on the template is unwieldy (imagine: 'hanning{num}.pdf'.format(num=my_mapping[num]), with a separate argument for each placeholder).
string.Formatter
The string standard library module contains a rarely used Formatter class. Using it looks like string.Formatter().format('hanning{num}.pdf', num=num). The template string uses the same syntax again. This is obviously clunkier than just calling .format on the string; the motivation is to allow users to subclass Formatter to define a different syntax for the template string.
All of the above approaches use a common "formatting language" (although string.Formatter allows changing it); there are many other things that can be put inside the {}. Explaining how it works is beyond the scope of this answer; please consult the documentation. Do keep in mind that literal { and } characters need to be escaped by doubling them up. The syntax is presumably inspired by C#.
The % operator
This is a legacy way to solve the problem, inspired by C and C++. It has been discouraged for a long time, but is still supported. It looks like 'hanning%s.pdf' % num, for simple cases. As you'd expect, literal '%' symbols in the template need to be doubled up to escape them.
It has some issues:
It seems like the conversion specifier (the letter after the %) should match the type of whatever is being interpolated, but that's not actually the case. Instead, the value is converted to the specified type, and then to string from there. This isn't normally necessary; converting directly to string works most of the time, and converting to other types first doesn't help most of the rest of the time. So 's' is almost always used (unless you want the repr of the value, using 'r'). Despite that, the conversion specifier is a mandatory part of the syntax.
Tuples are handled specially: passing a tuple on the right-hand side is the way to provide multiple arguments. This is an ugly special case that's necessary because we aren't using function-call syntax. As a result, if you actually want to format a tuple into a single placeholder, it must be wrapped in a 1-tuple.
Other sequence types are not handled specially, and the different behaviour can be a gotcha.
string.Template
The string standard library module contains a rarely used Template class. Instances provide substitute and safe_substitute methods that work similarly to the built-in .format (safe_substitute will leave placeholders intact rather than raising an exception when the arguments don't match). This should also be considered a legacy approach to the problem.
It looks like string.Template('hanning$num.pdf').substitute(num=num), and is inspired by traditional Perl syntax. It's obviously clunkier than the .format approach, since a separate class has to be used before the method is available. Braces ({}) can be used optionally around the name of the variable, to avoid ambiguity. Similarly to the other methods, literal '$' in the template needs to be doubled up for escaping.
I had a need for an extended version of this: instead of embedding a single number in a string, I needed to generate a series of file names of the form 'file1.pdf', 'file2.pdf' etc. This is how it worked:
['file' + str(i) + '.pdf' for i in range(1,4)]
You can make dict and substitute variables in your string.
var = {"name": "Abdul Jalil", "age": 22}
temp_string = "My name is %(name)s. I am %(age)s years old." % var
I would like to put an int into a string. This is what I am doing at the moment:
num = 40
plot.savefig('hanning40.pdf') #problem line
I have to run the program for several different numbers, so I'd like to do a loop. But inserting the variable like this doesn't work:
plot.savefig('hanning', num, '.pdf')
How do I insert a variable into a Python string?
See also
If you tried using + to concatenate a number with a string (or between strings, etc.) and got an error message, see How can I concatenate str and int objects?.
If you are trying to assemble a URL with variable data, do not use ordinary string formatting, because it is error-prone and more difficult than necessary. Specialized tools are available. See Add params to given URL in Python.
If you are trying to assemble a SQL query, do not use ordinary string formatting, because it is a major security risk. This is the cause of "SQL injection" which costs real companies huge amounts of money every year. See for example Python: best practice and securest way to connect to MySQL and execute queries for proper techniques.
If you just want to print (output) the string, you can prepare it this way first, or if you don't need the string for anything else, print each piece of the output individually using a single call to print. See How can I print multiple things (fixed text and/or variable values) on the same line, all at once? for details on both approaches.
Using f-strings:
plot.savefig(f'hanning{num}.pdf')
This was added in 3.6 and is the new preferred way.
Using str.format():
plot.savefig('hanning{0}.pdf'.format(num))
String concatenation:
plot.savefig('hanning' + str(num) + '.pdf')
Conversion Specifier:
plot.savefig('hanning%s.pdf' % num)
Using local variable names (neat trick):
plot.savefig('hanning%(num)s.pdf' % locals())
Using string.Template:
plot.savefig(string.Template('hanning${num}.pdf').substitute(locals()))
See also:
Fancier Output Formatting - The Python Tutorial
Python 3's f-Strings: An Improved String Formatting Syntax (Guide) - RealPython
With the introduction of formatted string literals ("f-strings" for short) in Python 3.6, it is now possible to write this with a briefer syntax:
>>> name = "Fred"
>>> f"He said his name is {name}."
'He said his name is Fred.'
With the example given in the question, it would look like this
plot.savefig(f'hanning{num}.pdf')
plot.savefig('hanning(%d).pdf' % num)
The % operator, when following a string, allows you to insert values into that string via format codes (the %d in this case). For more details, see the Python documentation:
printf-style String Formatting
You can use + as the normal string concatenation function as well as str().
"hello " + str(10) + " world" == "hello 10 world"
In general, you can create strings using:
stringExample = "someString " + str(someNumber)
print(stringExample)
plot.savefig(stringExample)
If you would want to put multiple values into the string you could make use of format
nums = [1,2,3]
plot.savefig('hanning{0}{1}{2}.pdf'.format(*nums))
Would result in the string hanning123.pdf. This can be done with any array.
Special cases
Depending on why variable data is being used with strings, the general-purpose approaches may not be appropriate.
If you need to prepare an SQL query
Do not use any of the usual techniques for assembling a string. Instead, use your SQL library's functionality for parameterized queries.
A query is code, so it should not be thought about like normal text. Using the library will make sure that any inserted text is properly escaped. If any part of the query could possibly come from outside the program in any way, that is an opportunity for a malevolent user to perform SQL injection. This is widely considered one of the important computer security problems, costing real companies huge amounts of money every year and causing problems for countless customers. Even if you think you know the data is "safe", there is no real upside to using any other approach.
The syntax will depend on the library you are using and is outside the scope of this answer.
If you need to prepare a URL query string
See Add params to given URL in Python. Do not do it yourself; there is no practical reason to make your life harder.
Writing to a file
While it's possible to prepare a string ahead of time, it may be simpler and more memory efficient to just write each piece of data with a separate .write call. Of course, non-strings will still need to be converted to string before writing, which may complicate the code. There is not a one-size-fits-all answer here, but choosing badly will generally not matter very much.
If you are simply calling print
The built-in print function accepts a variable number of arguments, and can take in any object and stringify it using str. Before trying string formatting, consider whether simply passing multiple arguments will do what you want. (You can also use the sep keyword argument to control spacing between the arguments.)
# display a filename, as an example
print('hanning', num, '.pdf', sep='')
Of course, there may be other reasons why it is useful for the program to assemble a string; so by all means do so where appropriate.
It's important to note that print is a special case. The only functions that work this way are ones that are explicitly written to work this way. For ordinary functions and methods, like input, or the savefig method of Matplotlib plots, we need to prepare a string ourselves.
Concatenation
Python supports using + between two strings, but not between strings and other types. To work around this, we need to convert other values to string explicitly: 'hanning' + str(num) + '.pdf'.
Template-based approaches
Most ways to solve the problem involve having some kind of "template" string that includes "placeholders" that show where information should be added, and then using some function or method to add the missing information.
f-strings
This is the recommended approach when possible. It looks like f'hanning{num}.pdf'. The names of variables to insert appear directly in the string. It is important to note that there is not actually such a thing as an "f-string"; it's not a separate type. Instead, Python will translate the code ahead of time:
>>> def example(num):
... return f'hanning{num}.pdf'
...
>>> import dis
>>> dis.dis(example)
2 0 LOAD_CONST 1 ('hanning')
2 LOAD_FAST 0 (num)
4 FORMAT_VALUE 0
6 LOAD_CONST 2 ('.pdf')
8 BUILD_STRING 3
10 RETURN_VALUE
Because it's a special syntax, it can access opcodes that aren't used in other approaches.
str.format
This is the recommended approach when f-strings aren't possible - mainly, because the template string needs to be prepared ahead of time and filled in later. It looks like 'hanning{}.pdf'.format(num), or 'hanning{num}.pdf'.format(num=num)'. Here, format is a method built in to strings, which can accept arguments either by position or keyword.
Particularly for str.format, it's useful to know that the built-in locals, globals and vars functions return dictionaries that map variable names to the contents of those variables. Thus, rather than something like '{a}{b}{c}'.format(a=a, b=b, c=c), we can use something like '{a}{b}{c}'.format(**locals()), unpacking the locals() dict.
str.format_map
This is a rare variation on .format. It looks like 'hanning{num}.pdf'.format_map({'num': num}). Rather than accepting keyword arguments, it accepts a single argument which is a mapping.
That probably doesn't sound very useful - after all, rather than 'hanning{num}.pdf'.format_map(my_dict), we could just as easily write 'hanning{num}.pdf'.format(**my_dict). However, this is useful for mappings that determine values on the fly, rather than ordinary dicts. In these cases, unpacking with ** might not work, because the set of keys might not be determined ahead of time; and trying to unpack keys based on the template is unwieldy (imagine: 'hanning{num}.pdf'.format(num=my_mapping[num]), with a separate argument for each placeholder).
string.Formatter
The string standard library module contains a rarely used Formatter class. Using it looks like string.Formatter().format('hanning{num}.pdf', num=num). The template string uses the same syntax again. This is obviously clunkier than just calling .format on the string; the motivation is to allow users to subclass Formatter to define a different syntax for the template string.
All of the above approaches use a common "formatting language" (although string.Formatter allows changing it); there are many other things that can be put inside the {}. Explaining how it works is beyond the scope of this answer; please consult the documentation. Do keep in mind that literal { and } characters need to be escaped by doubling them up. The syntax is presumably inspired by C#.
The % operator
This is a legacy way to solve the problem, inspired by C and C++. It has been discouraged for a long time, but is still supported. It looks like 'hanning%s.pdf' % num, for simple cases. As you'd expect, literal '%' symbols in the template need to be doubled up to escape them.
It has some issues:
It seems like the conversion specifier (the letter after the %) should match the type of whatever is being interpolated, but that's not actually the case. Instead, the value is converted to the specified type, and then to string from there. This isn't normally necessary; converting directly to string works most of the time, and converting to other types first doesn't help most of the rest of the time. So 's' is almost always used (unless you want the repr of the value, using 'r'). Despite that, the conversion specifier is a mandatory part of the syntax.
Tuples are handled specially: passing a tuple on the right-hand side is the way to provide multiple arguments. This is an ugly special case that's necessary because we aren't using function-call syntax. As a result, if you actually want to format a tuple into a single placeholder, it must be wrapped in a 1-tuple.
Other sequence types are not handled specially, and the different behaviour can be a gotcha.
string.Template
The string standard library module contains a rarely used Template class. Instances provide substitute and safe_substitute methods that work similarly to the built-in .format (safe_substitute will leave placeholders intact rather than raising an exception when the arguments don't match). This should also be considered a legacy approach to the problem.
It looks like string.Template('hanning$num.pdf').substitute(num=num), and is inspired by traditional Perl syntax. It's obviously clunkier than the .format approach, since a separate class has to be used before the method is available. Braces ({}) can be used optionally around the name of the variable, to avoid ambiguity. Similarly to the other methods, literal '$' in the template needs to be doubled up for escaping.
I had a need for an extended version of this: instead of embedding a single number in a string, I needed to generate a series of file names of the form 'file1.pdf', 'file2.pdf' etc. This is how it worked:
['file' + str(i) + '.pdf' for i in range(1,4)]
You can make dict and substitute variables in your string.
var = {"name": "Abdul Jalil", "age": 22}
temp_string = "My name is %(name)s. I am %(age)s years old." % var
In Python, the following code produces an error:
a = 'abc'
b = 1
print(a + b)
(The error is "TypeError: cannot concatenate 'str' and 'int' objects").
Why does the Python interpreter not automatically try using the str() function when it encounters concatenation of these types?
The problem is that the conversion is ambiguous, because + means both string concatenation and numeric addition. The following question would be equally valid:
Why does the Python interpreter not automatically try using the int() function when it encounters addition of these types?
This is exactly the loose-typing problem that unfortunately afflicts Javascript.
There's a very large degree of ambiguity with such operations. Suppose that case instead:
a = '4'
b = 1
print(a + b)
It's not clear if a should be coerced to an integer (resulting in 5), or if b should be coerced to a string (resulting in '41'). Since type juggling rules are transitive, passing a numeric string to a function expecting numbers could get you in trouble, especially since almost all arithmetic operators have overloaded operations for strings too.
For instance, in Javascript, to make sure you deal with integers and not strings, a common practice is to multiply a variable by one; in Python, the multiplication operator repeats strings, so '41' * 1 is a no-op. It's probably better to just ask the developer to clarify.
The short answer would be because Python is a strongly typed language.
This was a design decision made by Guido. It could have been one way or another really, concatenating str and int to str or int.
The best explanation, is still the one given by guido, you can check it here
The other answers have provided pretty good explanations, but have failed to mention that this feature is known a Strong Typing. Languages that perform implicit conversions are Weakly Typed.
Because Python does not perform type conversion when concatenating strings. This behavior is by design, and you should get in the habit of performing explicit type conversions when you need to coerce objects into strings or numbers.
Change your code to:
a = 'abc'
b = 1
print(a + str(b))
And you'll see the desired result.
Python would have to know what's in the string to do it correctly. There's an ambiguous case: what should '5' + 5 generate? A number or a string? That should certainly throw an error. Now to determine whether that situation holds, python would have to examine the string to tell. Should it do that every time you try to concatenate or add two things? Better to just let the programmer convert the string explicitly.
More generally, implicit conversions like that are just plain confusing! They're hard to predict, hard to read, and hard to debug.
That's just how they decided to design the language. Probably the rationale is that requiring explicit conversions to string reduces the likelihood of unintended behavior (e.g. integer addition if both operands happen to be ints instead of strings).
tell python that the int is a list to disambiguate the '+' operation.
['foo', 'bar'] + [5]
this returns: ['foo', 'bar', 5]